Added support for manage/unmanage snapshot in HNAS NFS driver.
This patch added functions that allow volume snapshots on HNAS
be managed by OpenStack, also, volume snapshots can be deleted
from OpenStack but still left in HNAS.
DocImpact
Implements: blueprint hnas-nfs-manage-unmanage-snapshot-support
Depends-On: I08175b031a65ea6ae5cec3c73d5312175f29c890
Change-Id: If06ecadeab814ff2f9420cee2e537ac0f71f0f9a