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: Conditionally mount flexvolume
**What this PR does / why we need it**:
See https://github.com/kubernetes/kubeadm/issues/476
**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 https://github.com/kubernetes/kubeadm/issues/476
```release-note
kubeadm: Fix a bug on some OSes where the kubelet tried to mount a volume path that is non-existent and on a read-only filesystem
```
/cc @luxas
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