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>.

Cinder Volume API changes for adding support for secrets in the future

**What this PR does / why we need it**:
Many of the in-tree volume sources support specification of secret(s) for specific volumes. This support is not present in cinder currently. 

This PR adds just the changes in the API, so work can be done in the future in the external openstack controller manager.

**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 #

**Special notes for your reviewer**:


**Release note**:

```release-note
NONE
```