Implements support for managing and unmanaging snapshots to
the HPE 3PAR FC and iSCSI drivers.
This patch now allows snapshots to be removed from OpenStack
management but still left on the 3PAR backend. Snapshots on
the 3PAR backend can also be managed by OpenStack.
DocImpact
Implements: blueprint 3par-manage-unmanage-snapshot
Change-Id: I7f5847cefc983726eac8b0eaa57b63ddb9078ebd