upstart: add rbdmap script

Upstart script for mapping / unmapping rbd device based on /etc/ceph/rbdmap file.
It does not mount or unmount filesystem, this part should be performed by _netdev option in fstab.

Signed-off-by: Laurent Barbe <laurent@ksperis.com>
Merge pull request #965 from ksperis/rbdmap.upstart

upstart: add rbdmap script

Reviewed-by: Sage Weil <sage@inktank.com>