Added a spec for volume type description change
Added a spec for volume type description change

The purpose of this spec is to address the issue of current volume
type's lacking of information. The spec lays out how to improve
the volume type by adding the description information and
allowing user to find out what is the default volume type.

Change-Id: I6d92cd327b60024ad7f067ddcfc32bf70952f58d

openstack/cinder-specs