Automatic merge from submit-queue (batch tested with PRs 49840, 54937, 54543). 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>.
Add self-hosted etcd API to kubeadm
**What this PR does / why we need it**:
This PR is part of a larger set that implements self-hosted etcd. This PR takes a first step by adding:
1. new API types in `cmd/kubeadm/app/apis` for configuring self-hosted etcd
2. new Go types in `cmd/kubeadm/app/phases/etcd/spec` used for constructing EtcdCluster CRDs for the etcd-operator. The reason we define these in trunk is because kubeadm cannot import `github.com/coreos/etcd-operator` as a dependency until it's in its own repo. Until then, we need to redefine the structs in our codebase.
**Which issue this PR fixes**:
https://github.com/kubernetes/kubeadm/issues/261https://github.com/kubernetes/kubeadm/issues/277
**Special notes for your reviewer**:
This is the first step PR in order to save reviewers from a goliath PR
**Release note**:
```release-note
NONE
```
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