Added copy-on-write cloning of images to volumes.

This enables COW cloning of images into rbd-backed Cinder volumes.

Change-Id: Ibb114c90f2d5a2d258b25155cc800ab7ce77adfc
Merge "Added copy-on-write cloning of images to volumes."
Merge git://github.com/puppetlabs/puppetlabs-glance