Automatic merge from submit-queue (batch tested with PRs 50213, 50707, 49502, 51230, 50848)

Refactor CephFS PV spec

**What this PR does / why we need it**:
refactor CephFS Volume Persistent Volume Spec so CephFS PV's SecretRef allows referencing a secret from a persistent volume in any namespace. This allows locating credentials for persistent volumes in namespaces other than the one containing the PVC.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes https://github.com/kubernetes-incubator/external-storage/issues/309

**Special notes for your reviewer**:
@kubernetes/sig-storage-api-reviews 

**Release note**:

```release-note
Allow CephFS PV to specify a namespace for secret
```