Add rc script for rbd map/unmap

Init script for mapping/unmapping rbd device on startup and shutdown.
On start, map rbd dev according to /etc/rbdmap, and force mount -a
On stop, umount file system depending on rbd and unmap all rbd
Since some distribution use symlink for /etc/mtab, the user-space attribute _netdev is not enough to umount file system before rbd dev.
(also concern: #1790)

Signed-off-by: Laurent Barbe <laurent@ksperis.com>
Merge pull request #378 from ceph/wip-init-rbd

Reviewed-by: Sage Weil <sage@inktank.com>