rbd: implement get_volume_stats()

Report raw capacity because replication level may change, and the
cluster may be used by more than one service. If the command fails for
any reason, return unknown capacity.

Change-Id: If6871124301b51e4768ea64c4d8e5109e0804db8
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge "rbd: implement get_volume_stats()"