Automatic merge from submit-queue (batch tested with PRs 64283, 67910, 67803, 68100). If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

Kubeadm Cert Renewal

**What this PR does / why we need it**:

adds explicit support for renewal of certificates via command

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes kubernetes/kubeadm#206

**Special notes for your reviewer**:
The targeted documentation is at kubernetes/website#9712

**Release note**:

```release-note
Adds the commands `kubeadm alpha phases renew <cert-name>`
```