Implements support for managing and unmanaging snapshots to
the HPE LeftHand driver.
This patch now allows snapshots to be removed from OpenStack
management but still left on the LeftHand backend. Snapshots
on the LeftHand backend can also be managed by OpenStack.
DocImpact
Implements: blueprint lefthand-manage-unmanage-snapshot
Change-Id: Ic0048fd5da437cf8caddc166cf60ec035def39aa
