Automatic merge from submit-queue
kubeadm: Use the v1.7 branch by default
**What this PR does / why we need it**:
Makes kubeadm use the v1.7 branch instead of v1.6
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
This should be merged right before the rc.0 is cut I guess
**Release note**:
```release-note
NONE
```
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