This patch implements the spec of reverting volume to
latest snapshot.
Related tempest and client patches:
[1] https://review.openstack.org/#/c/463906/
[2] https://review.openstack.org/#/c/464903/
APIImpact
DocImpact
Partial-Implements: blueprint revert-volume-to-snapshot
Change-Id: Ib20d749c2118c350b5fa0361ed1811296d518a17