Automatic merge from submit-queue. 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: Use the release-1.8 branch by default
**What this PR does / why we need it**:
In line with https://github.com/kubernetes/kubeadm/blob/master/docs/release-cycle.md, the default branch for kubeadm to deploy should be bumped right before the `rc.1`.
This can even be manually merged by the someone from the release team.
Adding do-not-merge until we're ready for the release candidate.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
fixes: https://github.com/kubernetes/kubeadm/issues/427
**Special notes for your reviewer**:
**Release note**:
```release-note
kubeadm: Use the release-1.8 branch by default
```
@kubernetes/sig-cluster-lifecycle-pr-reviews @kubernetes/kubernetes-release-managers
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