Add "--remote-source" option to "volume snapshot create" command
to support creating snapshot from an existing remote snapshot in
volume v2 (v2 only), also add the doc, unit tests and release note.
Change-Id: I9e5fad4f0db5b44d528eb6b930edbc816e392c3a
Implements: bp cinder-command-support
Closes-Bug: #1618676
Co-Authored-By: Sheel Rana <ranasheel2000@gmail.com>