Automatic merge from submit-queue (batch tested with PRs 45724, 48051, 46444, 51056, 51605)

Mount propagation in kubelet

Together with #45724 it implements mount propagation as proposed in https://github.com/kubernetes/community/pull/659

There is:

- New alpha annotation that allows user to explicitly set propagation mode for each `VolumeMount` in pod containers (to be replaced with real `VolumeMount.Propagation` field during beta) + validation + tests. "Private" is the default one (= no change to existing pods).

  I know about proposal for real API fields for alpha feature in https://docs.google.com/document/d/1wuoSqHkeT51mQQ7dIFhUKrdi3-1wbKrNWeIL4cKb9zU/edit, but it seems it's not implemented yet. It would save me quite lot of code and ugly annotation.

- Updated CRI API to transport chosen propagation to Docker.

- New `kubelet --experimental-mount-propagation` option to enable the previous bullet without modifying types.go (worked around with changing `KubeletDeps`... not nice, but it's better than adding a parameter to `NewMainKubelet` and removing it in the next release...)

```release-note
kubelet has alpha support for mount propagation. It is disabled by default and it is there for testing only. This feature may be redesigned or even removed in a future release.
```

@derekwaynecarr @dchen1107 @kubernetes/sig-node-pr-reviews