Automatic merge from submit-queue (batch tested with PRs 64593, 65117, 65629, 65827, 65686). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

kubeadm: Add a `v1alpha3` API

**What this PR does / why we need it**:
Adds a (now duplicated) v1alpha3 API. Equal to `v1alpha2` now, but will be changed later.
Hopefully we'll graduate this to `v1beta1` during the v1.12 cycle.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref: https://github.com/kubernetes/kubeadm/issues/911
Depends on:
 - [x] https://github.com/kubernetes/kubernetes/pull/65776
 - [x] https://github.com/kubernetes/kubernetes/pull/65628

**Special notes for your reviewer**:

These commits should be reviewed:
 - `Add a duplicated v1alpha3 API`: A pure directory copy of the `v1alpha2` folder, only changed the package name in the Go files
 - `Register the v1alpha3 API in the scheme, and update the roundtrip API tests`, started registering the new API and using it internally
 - `Automated bump from v1alpha2 references to v1alpha3`: Replaced all v1alpha2 references with v1alpha3

**Release note**:

```release-note
kubeadm: Add a `v1alpha3` API.
```
@kubernetes/sig-cluster-lifecycle-pr-reviews