Update volume-backed comment in _validate_flavor_image_nostatus
Update volume-backed comment in _validate_flavor_image_nostatus

This clarifies and updates the big comment in the volume-backed
case in the _validate_flavor_image_nostatus method since the
method is called by more than just server create. The method is
definitely tightly coupled with what calls it, so this tries to
clarify that.

Change-Id: I4b56458767e2712ad812db3b0901807d2c4aa848

klmitch/nova,openstack/nova,rahulunair/nova,openstack/nova,mahak/nova,openstack/nova,mahak/nova,rahulunair/nova,mahak/nova,rahulunair/nova,klmitch/nova,klmitch/nova,klmitch/nova