Automatic merge from submit-queue (batch tested with PRs 55908, 55829, 55293, 55653, 55665). 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>.
If mountPath is missing, prefix with root dir.
If `mountPath` is not absolute, add the os-specific root directory as a prefix.
Ref: https://github.com/kubernetes/kubernetes/pull/51240https://groups.google.com/forum/#!topic/kubernetes-sig-storage/k_0Wr2kYkpU
@thockin @saad-ali @andyzhangx
Note to @thockin I left the validation in place, in seems like it is a net win, since it will give a decent
error message to most people, but the fall-back defaulting is there if it doesn't catch 'c:'
I'm happy to rip out the validation if that is preferable to everyone. Let me know.
```release-note
If a non-absolute mountPath is passed to the kubelet, prefix it with the appropriate root path.
```
01c74145c7