kubeadm: 1.12: missleading errors when passing --config

NOTE: this page needs an update: https://kubernetes.io/docs/setup/independent/high-availability/

ref: https://github.com/kubernetes/website/issues/10450

users of kubeadm 1.12.x are reporting that they see errors when they do actions like:

kubeadm alpha phase kubelet write-env-file --config kubeadm-config.yaml

didn't recognize types with GroupVersionKind: [kubeadm.k8s.io/v1alpha3, Kind=ClusterConfiguration]

this is due to the fact that they are passing only ClusterConfiguration and not combining with InitConfiguration:

kind: ClusterConfiguration
apiVersion: kubeadm.k8s.io/v1alpha3
kubernetesVersion: v1.12.1
---
kind: InitConfiguration
apiVersion: kubeadm.k8s.io/v1alpha3

/kind bug /priority important-soon /assign @fabriziopandini @rosti @neolit123 /cc @detiber