Add default_volume_type as cinder api parameter

Adds a parameter to set the default_volume_type for the cinder api.
Cinder-api uses this value if in CLI the parameter '--volume-type' is not
given.

Change-Id: I6cdad0a90d249bdaa2729d201cf10d3b16dd96a1
(cherry picked from commit cd7391c5e2567a081d5a214a80cb773caa1ae2c6)