Automatic merge from submit-queue (batch tested with PRs 63588, 63806). 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 - do not generate etcd ca/certs for external etcd
**What this PR does / why we need it**:
Currently we generate an etcd CA and certificates even if we are specifying an external etcd cluster when running `kubeadm init`, this PR changes this behavior to skip generating the etcd CA and certificates if configured for an external etcd cluster.
**Which issue(s) this PR fixes**
Fixes https://github.com/kubernetes/kubeadm/issues/807
**Release note**:
```release-note
kubeadm will no longer generate an unused etcd CA and certificates when configured to use an external etcd cluster.
```
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