mon: ensure socker is purged when mon is stopped

mon: ensure socker is purged when mon is stopped

On containerized deployment, if a mon is stopped, the socket is not
purged and can cause failure when a cluster is redeployed after the
purge playbook has been run.

Typical error:

```
fatal: [osd0]: FAILED! => {}

MSG:

'dict object' has no attribute 'osd_pool_default_pg_num'
```

the fact is not set because of this previous failure earlier:

```
ok: [mon0] => {
    "changed": false,
    "cmd": "docker exec ceph-mon-mon0 ceph --cluster test daemon mon.mon0 config get osd_pool_default_pg_num",
    "delta": "0:00:00.217382",
    "end": "2018-07-09 22:25:53.155969",
    "failed_when_result": false,
    "rc": 22,
    "start": "2018-07-09 22:25:52.938587"
}

STDERR:

admin_socket: exception getting command descriptions: [Errno 111] Connection refused

MSG:

non-zero return code
```

This failure happens when the ceph-mon service is stopped, indeed, since
the socket isn't purged, it's a leftover which is confusing the process.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>