cluster-api: `Machine.Spec.Version` should not be optional

What steps did you take and what happened: If Machine.Spec.Version is optional, so Machine.Spec.Version may not be configured. This will result in the following errors: the logs in kubeadm-bootstrap:

E0722 07:05:46.218660       1 kubeadmconfig_controller.go:481] controller-runtime/manager/controller/kubeadmconfig "msg"="Failed to marshal join configuration" "error"="version cannot be empty" "kind"="Machine" "name"="cmcluster-md-0-75ff4bf755-bcpjc" "namespace"="default" "reconciler group"="bootstrap.cluster.x-k8s.io" "reconciler kind"="KubeadmConfig" "version"="8597463"
E0722 07:05:46.219322       1 controller.go:302] controller-runtime/manager/controller/kubeadmconfig "msg"="Reconciler error" "error"="version cannot be empty" "name"="cmcluster-md-0-pbfmk" "namespace"="default" "reconciler group"="bootstrap.cluster.x-k8s.io" "reconciler kind"="KubeadmConfig"

and the machine status:

NAME                              PROVIDERID                                          PHASE     VERSION
cmcluster-control-plane-pdlvx     openstack:///510b45ab-b5e0-4dbc-892b-6f00b59c331c   Running   v1.19.4
cmcluster-md-0-75ff4bf755-bcpjc                                                       Pending

What did you expect to happen:

Anything else you would like to add: [Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

/kind bug /area api /area machine