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
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