Fix default_volume_type for Cinder multi backend

The default_volume_type option doesn't support comma separated
arguments. We can only select one default volume type.
We have a variable that contains the default volume type, so we simply use it.

Closes-Bug: 1342569

Change-Id: Ibd080442f00f3d446ec374fc5906f0778d449007
Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge "Fix default_volume_type for Cinder multi backend"