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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)*:
Fixeskubernetes/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
```
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