ctdb/ceph: Add optional namespace support for mutex helper

ctdb/ceph: Add optional namespace support for mutex helper

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15665

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>
(cherry picked from commit d8c52995f68fe088dd2174562faee69ed1c95edd)