Automatic merge from submit-queue (batch tested with PRs 42365, 42429, 41770, 42018, 35055)

kubeadm: Add --cert-dir, --cert-altnames instead of --api-external-dns-names

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

 - For the beta kubeadm init UX, we need this change
 - Also adds the `kubeadm phase certs selfsign` command that makes the phase invokable independently

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

This PR depends on https://github.com/kubernetes/kubernetes/pull/41897

**Release note**:

```release-note
```
@dmmcquay @pires @jbeda @errordeveloper @mikedanese @deads2k @liggitt