Automatic merge from submit-queue (batch tested with PRs 58259, 58664). 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>.

Support out-of-tree / external cloud providers

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

Currently kubeadm fails in preflight check. We should allow external cloud providers
```
[preflight] Starting the kubelet service
cloudprovider: Invalid value: "external": cloudprovider not supported
```

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
Added support for external cloud providers in kubeadm
```