Automatic merge from submit-queue (batch tested with PRs 62464, 62947). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
make API.ControlPlaneEndpoint accept IP
**What this PR does / why we need it**:
This PR implements one of the actions defined by https://github.com/kubernetes/kubeadm/issues/751 (checklist form implementing HA in kubeadm).
With this PR, the `API.ControlPlaneEndpoint` value in the kubeadm MasterConfiguration file now accepts both DNS and IP.
The `API.ControlPlaneEndpoint` should be used to set a stable IP address for the control plane; in an HA configuration, this should be the load balancer address (no matter if identified by a DNS name or by a stable IP).
**Special notes for your reviewer**:
/CC @timothysc
This PR is the same of https://github.com/kubernetes/kubernetes/pull/62667, that I closed by error 😥
**Release note**:
```release-note
NONE
```
Nb. first https://github.com/kubernetes/kubernetes/pull/62667 already has the release note
d23ad1f894