rbd: clean up unused parents of clones on deletion

In order to make snapshotting ephemeral disks stored in rbd efficient
and avoid any local disk use, nova will tell glance about an rbd
snapshot's location rather than uploading data.

Since rbd images with snapshots cannot be deleted, nova cannot remove
the original disk in this case. Instead, nova renames the original rbd
image so glance can determine that it is no longer used by
nova. Glance can then delete this rbd image once its snapshots are
deleted.

To effect this, modify the rbd store's backend to recursively delete
any parents that have a special string in their name. This has
no chance of name collision from other openstack use since rbd images
are otherwise only named with uuids.

To allow glance to do this, it needs write access to the ceph pool
used by nova ephemeral disks.

DocImpact
Partial-Bug: #1346525
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>