cluster-api: Kubeadm Bootstrap Controller crashes when passed key material via File field

/kind bug

What steps did you take and what happened:

1. I created an external etcd cluster and made a copy of the apiserver-etcd-client.{crt,key} files.
2. I added the contents of these files to the kubeadmConfigSpec field of the KubeadmControlPlane object in a manifest for a workload cluster.
3. I applied the workload cluster manifest to the management cluster. After seeing no activity for a few minutes, I checked the logs of the various controllers, and saw a crash in the logs for the Kubeadm bootstrap controller.

What did you expect to happen: I expected the Kubeadm bootstrap controller to create the files with the specified content at the specified location.

Anything else you would like to add: Here is the kubeadmConfigSpec field I used (note the private key material has been redacted):

spec:
  infrastructureTemplate:
    apiVersion: infrastructure.cluster.x-k8s.io/v1alpha3
    kind: AWSMachineTemplate
    name: capi-etcd-control-plane
  kubeadmConfigSpec:
    clusterConfiguration:
      apiServer:
        extraArgs:
          cloud-provider: aws
      controllerManager:
        extraArgs:
          cloud-provider: aws
          configure-cloud-routes: "false"
      etcd:
        external:
          endpoints:
            - https://ip-10-45-16-154.us-west-2.compute.internal:2379
            - https://ip-10-45-53-148.us-west-2.compute.internal:2379
            - https://ip-10-45-84-141.us-west-2.compute.internal:2379
          caFile: /etc/kubernetes/pki/etcd/ca.crt
          certFile: /etc/kubernetes/pki/apiserver-etcd-client.crt
          keyFile: /etc/kubernetes/pki/apiserver-etcd-client.key
    initConfiguration:
      nodeRegistration:
        kubeletExtraArgs:
          cloud-provider: aws
        name: '{{ ds.meta_data.local_hostname }}'
    joinConfiguration:
      nodeRegistration:
        kubeletExtraArgs:
          cloud-provider: aws
        name: '{{ ds.meta_data.local_hostname }}'
    files:
      - path: /etc/kubernetes/pki/apiserver-etcd-client.crt
        content: |
          -----BEGIN CERTIFICATE-----
          MIIC+TCCAeGgAwIBAgIIR8jHKnG7IGcwDQYJKoZIhvcNAQELBQAwEjEQMA4GA1UE
          AxMHZXRjZC1jYTAeFw0yMDA0MDEyMzI0MjdaFw0yMTA0MDEyMzI1MDdaMD4xFzAV
          BgNVBAoTDnN5c3RlbTptYXN0ZXJzMSMwIQYDVQQDExprdWJlLWFwaXNlcnZlci1l
          dGNkLWNsaWVudDCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBANIe194P
          oqtyvXnM7a3cpTzzWVWL+EEBCvznBRNVgjQ4jMWYKR3Coq71x11zk/dox/6FGS0S
          wwaCBMIvnc7Mh5l9Tr9w4ZQdn5WXed0eqhNq/Eo7L0KZx0W7EtYH5t+ogQ8tVsIl
          4kU3zVAmeBFuSpXz3/JOq9Tbx9qW2NewbYBosiWrm+r+BEyAD9iwY0Melm7wJzMe
          T+vYKwRXhUbPxdFod6x9dHp/bHoxXaQLeVlwjHauvFnc1Q7B4AmoGjpGJ4KR85pM
          z+EAilatQ8yAt57e05yME7hOgO6MFA0CLkOQjmqhFzlLPknBz32oLc/cCftFKlnN
          5YfAutmbtBLYNAsCAwEAAaMnMCUwDgYDVR0PAQH/BAQDAgWgMBMGA1UdJQQMMAoG
          CCsGAQUFBwMCMA0GCSqGSIb3DQEBCwUAA4IBAQCQ20D/Z0+1yMyoqweetcY9j7gw
          CVj5NI477I/g+TBqIUb47+0VxPKlimKGH9yzcNNU41EAOVX+tbAhORot4YOe5zp0
          VSkJyHt7npI+K+sAtkdUQuC9K1730jCM1XjReuu65vc6dKxdagFAFi0m3EzrjHwb
          /aI4rhL8upszNh6UtQlP9EAoJMSwC8VSNdc0nE3Ta/otQNd+8TJui3MsSa2gIRaA
          zL9Ztl/yH/Gj/2u4nQuXE1iCE/aWZEfguJwb7756GMaVuDSywdB0oY+HTZhbuJyE
          wWKFQ2NF+P3UXMaLlxjMrkttDxENrx47Fh9R1q/hXjr2KA9lE+2N2+HbqEYP
          -----END CERTIFICATE-----
      - path: /etc/kubernetes/pki/apiserver-etcd-client.key
        content: |
          <redacted>
  replicas: 3
  version: v1.17.3

Here is the output of kubectl logs against the bootstrap controller:

I0402 18:47:06.669455       1 listener.go:44] controller-runtime/metrics "msg"="metrics server is starting to listen"  "addr"="127.0.0.1:8080"
I0402 18:47:06.669770       1 main.go:132] setup "msg"="starting manager"  
I0402 18:47:06.670001       1 leaderelection.go:242] attempting to acquire leader lease  capi-kubeadm-bootstrap-system/kubeadm-bootstrap-manager-leader-election-capi...
I0402 18:47:06.670140       1 internal.go:356] controller-runtime/manager "msg"="starting metrics server"  "path"="/metrics"
I0402 18:47:24.066941       1 leaderelection.go:252] successfully acquired lease capi-kubeadm-bootstrap-system/kubeadm-bootstrap-manager-leader-election-capi
I0402 18:47:24.068022       1 controller.go:164] controller-runtime/controller "msg"="Starting EventSource"  "controller"="kubeadmconfig" "source"={"Type":{"metadata":{"creationTimestamp":null},"spec":{},"status":{}}}
I0402 18:47:24.168701       1 controller.go:164] controller-runtime/controller "msg"="Starting EventSource"  "controller"="kubeadmconfig" "source"={"Type":{"metadata":{"creationTimestamp":null},"spec":{"clusterName":"","bootstrap":{},"infrastructureRef":{}},"status":{"bootstrapReady":false,"infrastructureReady":false}}}
I0402 18:47:24.269613       1 controller.go:164] controller-runtime/controller "msg"="Starting EventSource"  "controller"="kubeadmconfig" "source"={"Type":{"metadata":{"creationTimestamp":null},"spec":{"controlPlaneEndpoint":{"host":"","port":0}},"status":{"infrastructureReady":false,"controlPlaneInitialized":false}}}
I0402 18:47:24.370272       1 controller.go:171] controller-runtime/controller "msg"="Starting Controller"  "controller"="kubeadmconfig"
I0402 18:47:24.370663       1 controller.go:190] controller-runtime/controller "msg"="Starting workers"  "controller"="kubeadmconfig" "worker count"=10
I0402 18:47:24.472482       1 kubeadmconfig_controller.go:267] controllers/KubeadmConfig "msg"="ConfigOwner is not a control plane Machine. If it should be a control plane, add the label `cluster.x-k8s.io/control-plane: \"\"` to the Machine" "kind"="Machine" "kubeadmconfig"={"Namespace":"default","Name":"capi-etcd-md-0-dtmjt"} "name"="capi-etcd-md-0-7d49f97f5c-ph98v" "version"="6204997" 
I0402 18:47:24.472816       1 kubeadmconfig_controller.go:267] controllers/KubeadmConfig "msg"="ConfigOwner is not a control plane Machine. If it should be a control plane, add the label `cluster.x-k8s.io/control-plane: \"\"` to the Machine" "kind"="Machine" "kubeadmconfig"={"Namespace":"default","Name":"capi-etcd-md-0-c9w2d"} "name"="capi-etcd-md-0-7d49f97f5c-bnqvb" "version"="6205014" 
I0402 18:47:24.474533       1 kubeadmconfig_controller.go:267] controllers/KubeadmConfig "msg"="ConfigOwner is not a control plane Machine. If it should be a control plane, add the label `cluster.x-k8s.io/control-plane: \"\"` to the Machine" "kind"="Machine" "kubeadmconfig"={"Namespace":"default","Name":"capi-etcd-md-0-fcwxx"} "name"="capi-etcd-md-0-7d49f97f5c-9fpvk" "version"="6205016" 
I0402 18:47:24.579069       1 kubeadmconfig_controller.go:298] controllers/KubeadmConfig "msg"="Creating BootstrapData for the init control plane" "kind"="Machine" "kubeadmconfig"={"Namespace":"default","Name":"capi-etcd-control-plane-2sqs7"} "name"="capi-etcd-control-plane-7jgjz" "version"="6204332" 
E0402 18:47:24.681536       1 runtime.go:78] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 267 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic(0x14f8c80, 0x245b2f0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/runtime/runtime.go:74 +0xa3
k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/runtime/runtime.go:48 +0x82
panic(0x14f8c80, 0x245b2f0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
sigs.k8s.io/cluster-api/util/secret.(*Certificate).AsFiles(...)
	/workspace/util/secret/certificates.go:318
sigs.k8s.io/cluster-api/util/secret.Certificates.AsFiles(0xc000467c50, 0x5, 0x6, 0x7, 0xc0005aaf30, 0xc000170e00)
	/workspace/util/secret/certificates.go:361 +0x52d
sigs.k8s.io/cluster-api/bootstrap/kubeadm/internal/cloudinit.NewInitControlPlane(0xc0001b4460, 0xc0001b4460, 0x6, 0x18d9280, 0xc0000ac060, 0x18f3480)
	/workspace/bootstrap/kubeadm/internal/cloudinit/controlplane_init.go:55 +0x7e
sigs.k8s.io/cluster-api/bootstrap/kubeadm/controllers.(*KubeadmConfigReconciler).handleClusterNotInitialized(0xc0000fd700, 0x18d9280, 0xc0000ac060, 0xc0006d9c38, 0xc000114200, 0x0, 0x0, 0x0)
	/workspace/bootstrap/kubeadm/controllers/kubeadmconfig_controller.go:355 +0x79f
sigs.k8s.io/cluster-api/bootstrap/kubeadm/controllers.(*KubeadmConfigReconciler).Reconcile(0xc0000fd700, 0xc00049d039, 0x7, 0xc00049f000, 0x1d, 0xc000406c00, 0x0, 0x0, 0x0)
	/workspace/bootstrap/kubeadm/controllers/kubeadmconfig_controller.go:240 +0x126a
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0xc0001bc540, 0x1548d80, 0xc000529b00, 0xc000325d00)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:256 +0x162
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem(0xc0001bc540, 0x0)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:232 +0xcb
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).worker(0xc0001bc540)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:211 +0x2b
k8s.io/apimachinery/pkg/util/wait.JitterUntil.func1(0xc0003dfb50)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:152 +0x5e
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc0003dfb50, 0x3b9aca00, 0x0, 0x45ed01, 0xc0004c23c0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:153 +0xf8
k8s.io/apimachinery/pkg/util/wait.Until(0xc0003dfb50, 0x3b9aca00, 0xc0004c23c0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:88 +0x4d
created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func1
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:193 +0x328
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x139466d]

goroutine 267 [running]:
k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/runtime/runtime.go:55 +0x105
panic(0x14f8c80, 0x245b2f0)
	/usr/local/go/src/runtime/panic.go:679 +0x1b2
sigs.k8s.io/cluster-api/util/secret.(*Certificate).AsFiles(...)
	/workspace/util/secret/certificates.go:318
sigs.k8s.io/cluster-api/util/secret.Certificates.AsFiles(0xc000467c50, 0x5, 0x6, 0x7, 0xc0005aaf30, 0xc000170e00)
	/workspace/util/secret/certificates.go:361 +0x52d
sigs.k8s.io/cluster-api/bootstrap/kubeadm/internal/cloudinit.NewInitControlPlane(0xc0001b4460, 0xc0001b4460, 0x6, 0x18d9280, 0xc0000ac060, 0x18f3480)
	/workspace/bootstrap/kubeadm/internal/cloudinit/controlplane_init.go:55 +0x7e
sigs.k8s.io/cluster-api/bootstrap/kubeadm/controllers.(*KubeadmConfigReconciler).handleClusterNotInitialized(0xc0000fd700, 0x18d9280, 0xc0000ac060, 0xc0006d9c38, 0xc000114200, 0x0, 0x0, 0x0)
	/workspace/bootstrap/kubeadm/controllers/kubeadmconfig_controller.go:355 +0x79f
sigs.k8s.io/cluster-api/bootstrap/kubeadm/controllers.(*KubeadmConfigReconciler).Reconcile(0xc0000fd700, 0xc00049d039, 0x7, 0xc00049f000, 0x1d, 0xc000406c00, 0x0, 0x0, 0x0)
	/workspace/bootstrap/kubeadm/controllers/kubeadmconfig_controller.go:240 +0x126a
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0xc0001bc540, 0x1548d80, 0xc000529b00, 0xc000325d00)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:256 +0x162
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem(0xc0001bc540, 0x0)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:232 +0xcb
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).worker(0xc0001bc540)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:211 +0x2b
k8s.io/apimachinery/pkg/util/wait.JitterUntil.func1(0xc0003dfb50)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:152 +0x5e
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc0003dfb50, 0x3b9aca00, 0x0, 0x45ed01, 0xc0004c23c0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:153 +0xf8
k8s.io/apimachinery/pkg/util/wait.Until(0xc0003dfb50, 0x3b9aca00, 0xc0004c23c0)
	/go/pkg/mod/k8s.io/apimachinery@v0.17.2/pkg/util/wait/wait.go:88 +0x4d
created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func1
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.5.1/pkg/internal/controller/controller.go:193 +0x328

Environment:

• Cluster-api-provider-aws version: 0.5.0
• Kubernetes version: (use kubectl version): 1.17.4 (client), 1.16.4 (server)
• OS (e.g. from /etc/os-release): Ubuntu 18.04.4 (using CAPA AMI)