Enable trove to specify cinder volume_type when creating a volume

The volume_type is configurable via the trove-taskmanager.conf.
By default the volume_type is None.

Change-Id: I23dab1a6223803b03f2857842150cf1433af6ad0
Implements: blueprint cinder-volume-type
Merge "Enable trove to specify cinder volume_type when creating a volume"