Nexenta: Use base VolumeDriver migrate_volume
Nexenta: Use base VolumeDriver migrate_volume

NotImplemented is a constant for boolean comparison,
where this code actually meant NotImplementedError().

But this method is not needed, it should just use
the base migrate_volume() which returns (False, None)
rather than raising this exception.

Change-Id: I02f2dfb32a90b702b4017d5c9a47334732ff9f1e

phenoxim/cinder,mahak/cinder,cloudbase/cinder,eharney/cinder,mahak/cinder,NetApp/cinder,Datera/cinder,Datera/cinder,ge0rgi/cinder,openstack/cinder,openstack/cinder,NetApp/cinder,cloudbase/cinder,Hybrid-Cloud/cinder,Hybrid-Cloud/cinder,eharney/cinder,phenoxim/cinder,j-griffith/cinder,j-griffith/cinder