Automatic merge from submit-queue (batch tested with PRs 64127, 63895, 64066, 64215, 64202). 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: Refactor the .Etcd substruct in the v1alpha2 API

**What this PR does / why we need it**:
Splits the monolithic `.Etcd` struct with all the options as fields to a more modular and clear design with two sub-structs for the different modes of hosting etcd we support.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Part of kubernetes/community#2131

**Special notes for your reviewer**:
Depends on:
 - [x] https://github.com/kubernetes/kubernetes/pull/63917

Follows up: https://github.com/kubernetes/kubernetes/pull/63871
TODO: I still need to write unit tests for this.

**Release note**:

```release-note
[action required] kubeadm: The `:Etcd` struct has been refactored in the v1alpha2 API. All the options now reside under either `.Etcd.Local` or `.Etcd.External`. Automatic conversions from the v1alpha1 API are supported.
```
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