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kubeadm join the cluster with pre-existing client certificate if provided

**What this PR does / why we need it**:
support `kubeadm join` with a pre-existing client certificate

**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 kubernetes/kubeadm#945

**Special notes for your reviewer**:
/cc @luxas @timothysc  @kubernetes/sig-cluster-lifecycle-pr-reviews 

**Release note**:

```release-note
kubeadm now can join the cluster with pre-existing client certificate if provided
```