doc: link to ephemeral rbd patches

These are necessary for using ephemeral rbd with Havana and Icehouse.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
doc: note cinder multi-backend restriction

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
doc: clarify openstack cow sentence

Missing subject, and now applies to disks in general.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1836 from ceph/wip-doc-openstack

doc: Clarifications to Ceph Block Device and OpenStack documentation.