Add default_volume_type to cinder config

This allows the administrator of a cloud with multiple backends to
define which volume is the default when a user creats a volume without
specifying a volume type.

Change-Id: I1bb94dc1283e50645e4aece600acc52ae17db56b
Co-Authored-By: Craige McWhirter <craige.mcwhirter@anchor.com.au>
Closes-Bug: #1389292