Adding support for manage/unmanage snapshots in Hitachi HNAS
driver. In order to manage a snapshot, the admin should provide the
snapshot size in "--driver-options" parameter.
Also, updating tempest tests for manage/unmanage snapshots to include
the required driver option.
DocImpact
Implements: blueprint hnas-manage-unmanage-snapshot-support
Change-Id: I93e56dda5cbe8d3dbe142d773f93d03a0c126d2f