Copy-on-write cloning for RBD-backed disks

Currently RBD-backed ephemeral disks are created by downloading an image from
Glance to a local file, then uploading that file into RBD. Even if the file is
cached, uploading may take a long time, since 'rbd import' is synchronous and
slow. If the image is already stored in RBD by Glance, there's no need for any
local copies - it can be cloned to a new image for a new disk without copying
the data at all.

blueprint rbd-clone-image-handler
Change-Id: Ie71699e845e14251233efc993a690713f1118c31
Merge "Copy-on-write cloning for RBD-backed disks"