mon: create fail_mds_gid() helper; make 'ceph mds rm ...' more generic

Take a gid or a rank or a name.  Use a nicer helper.

Signed-off-by: Sage Weil <sage@inktank.com>
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>
mds: gracefully exit if newer gid replaces us by name

If 'mds enforce unique name' is set, and another MDS with the same name
kicks us out of the MDSMap, gracefully exit instead of respawning and
fighting over our position.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>
deb:  Add ceph-coverage to ceph-test deb package

Teuthology uses the ceph-coverage script extensively
and expects it to be installed by the ceph task.  Add
the script to the ceph-test debian package so that it
gets installed for that use case.

Signed-off-by: Sam Lang <sam.lang@inktank.com>