rbd: allow configuration of rados from the rbd filename

rbd: allow configuration of rados from the rbd filename

The new format is rbd:pool/image[@snapshot][:option1=value1[:option2=value2...]]
Each option is used to configure rados, and may be any Ceph option, or "conf".
The "conf" option specifies a Ceph configuration file to read.

This allows rbd volumes from more than one Ceph cluster to be used by
specifying different monitor addresses, as well as having different
logging levels or locations for different volumes.

Reviewed-by: Christian Brunner <chb@muc.de>
Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>