ctdb/ceph: Add optional namespace support for mutex helper
RADOS objects within a pool can be associated to a namespace for
logical separation. librados already provides an API to configure
such a namespace with respect to a context. Make use of it as an
optional argument to the helper binary.
Pair-Programmed-With: Anoop C S <anoopcs@samba.org> Signed-off-by: Günther Deschner <gd@samba.org> Reviewed-by: Günther Deschner <gd@samba.org> Reviewed-by: David Disseldorp <ddiss@samba.org>