mon: enforce unique name in mdsmap

mon: enforce unique name in mdsmap

Add 'mds enforce unique name' option, defaulting to true.

If set, when an MDS boots, it will kick any previous mds with the same
name from the mdsmap.  This is possibly less confusing for users.  If
an mds daemon restarts, it will immediately replace its previous
instantiation.

Two misconfigured daemons running with the same name will fight over
the same role.

Fixes: #3857
Signed-off-by: Sage Weil <sage@inktank.com>