Enable cloning for rbd-backed ephemeral disks

Currently when using rbd as an image backend, nova downloads the glance
image to local disk and then copies it again into rbd. This can be very
slow for large images, and wastes bandwidth as well as disk space.

When the glance image is stored in the same ceph cluster, the data is
being pulled out and pushed back in unnecessarily. Instead, create a
copy-on-write clone of the image. This is fast, and does not depend on
the size of the image. Instead of taking minutes, booting takes seconds,
and is not limited by the disk copy.

Add some rbd utility functions from cinder to support cloning and let
the rbd imagebackend rely on librbd instead of the rbd command line tool
for checking image existence.

Adds a new clone() method to the image backend, so backends like rbd can
make optimizations like this. Try to use clone() for the root disk when
it comes from an image, but fall back to fetch_to_raw() if clone()
fails.

Instead of calling disk.get_disk_size() directly from
verify_base_size(), which assumes the disk is stored locally, add a new
method that is overridden by the Rbd subclass to get the disk size.

DocImpact

Implements: blueprint rbd-clone-image-handler
Closes-Bug: 1226351
Co-Authored-By: Josh Durgin <josh.durgin@inktank.com>
Signed-Off-By: Josh Durgin <josh.durgin@inktank.com>
Signed-Off-By: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Signed-Off-By: Dmitry Borodaenko <dborodaenko@mirantis.com>
Change-Id: I0f50659b54a92fc21086990be8925ea15008569a
Merge "Enable cloning for rbd-backed ephemeral disks"