Automatic merge from submit-queue (batch tested with PRs 50713, 47660, 51198, 51159, 51195)

kubeadm selfhosting CLI improvements

**What this PR does / why we need it**:
This PR implements three small improvements for kubeadm selfhosting CLI:
- `kubeadm phase selfhosting` should be `kubeadm phase self-hosting from-static-pods`
- add `--cert-dir`flag
- add `--config`flag and manage MixedArguments conditions

**Which issue this PR fixes**:
fixes [#397](https://github.com/kubernetes/kubeadm/issues/397) - `kubeadm phase selfhosting` should be `kubeadm phase self-hosting from-static-pods`

**Special notes for your reviewer**:
cc @luxas
cc @jamiehannaford