Merge pull request #41897 from luxas/kubeadm_secure_controlplane

Automatic merge from submit-queue (batch tested with PRs 41701, 41818, 41897, 41119, 41562)

kubeadm: Secure the control plane communication and add the kubeconfig phase command

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

This generates kubeconfig files for the controller-manager and the scheduler, ref: https://github.com/kubernetes/kubeadm/issues/172

The second commit adds the `kubeadm alpha phase kubeconfig` command as described in the design doc: https://github.com/kubernetes/kubeadm/pull/156

**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**:

@dmmcquay What kind of tests would you like for the kubeconfig phase command?

**Release note**:

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

hack/verify-flags/known-flags.txt

@@ -74,6 +74,7 @@ cleanup-iptables
 client-ca-file
 client-certificate
 client-key
+client-name
 clientset-api-path
 clientset-name
 clientset-only