Automatic merge from submit-queue (batch tested with PRs 55757, 55741, 55817, 55834, 51530). 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>.
Refactor iSCSI PV spec to use SecretReference
**What this PR does / why we need it**:
Use SecretReference in iSCSI PV spec
**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/321
**Special notes for your reviewer**:
@liggitt
**Release note**:
```release-note
iSCSI Persistent Volume Sources can now reference CHAP Secrets in namespaces other than the namespace of the bound Persistent Volume Claim
```