Allow Glance to COW images to Cinder Volumes

This bypasses will allow glance to bypass the ownloading
of the image to a volume and replaces it by doing a direct
COW within Ceph and other storage models that take advantage of it.

Change-Id: Idefdfe440bf07a80fe3db9ea91489ebb31ceaeb3
Merge remote-tracking branch 'upstream/master' into icehouse