[cinder][drivers] stable branch backports

Hello Argonauts, especially vendor driver maintainers:

At last week's midcycle meeting, we began a discussion of running third-party CI on stable branch changes. While that continues *not* to be required (it will be re-visited at the Z PTG), we did agree to the following:

When a vendor driver patch backport is proposed, we would like to see a clear statement on the gerrit review that the patch has been tested in an appropriate environment.

This shouldn't be a big deal because presumably you've done local testing with your backend to ensure that the code works as expected in a stable branch; we're simply asking that this be documented on the backport.

cheers, brian