Automatic merge from submit-queue (batch tested with PRs 40707, 40698)

kubeadm: Default to control plane v1.6.0-alpha.1 and using RBAC

Also use constants for authz modes

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

Defaults to v1.6.0-alpha.1 (will be cut later today) because the certificates API group has been upgraded to beta, so `kubeadm join` at HEAD doesn't work on a `v1.5` cluster anyway.

By defaulting to v1.6.0-alpha.1, we can focus totally on v1.6 for kubeadm at HEAD, we don't support other versions in the upcoming v1.6 kubeadm release because of the alpha -> beta upgrades.

**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**:

**Release note**:

```release-note
NONE
```

@mikedanese @pires