add ability to clone images

add ability to clone images

Given the backend location from Glance, drivers can determine
whether they can clone or otherwise efficiently create a volume
from the image without downloading all the data from Glance.

For now implement cloning for the RBD driver. There's already a
Glance backend that stores images as RBD snapshots, so they're
ready to be cloned into volumes. Fall back to copying all the
data if cloning is not possible.

Change-Id: I71a8172bd22a5bbf64d4c68631630125fcc7fd34
Implements: blueprint efficient-volumes-from-images
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>