rgw: run the handler on a mon host

In case the admin wasn't copied over to the node this command would
fail. So it's safer to run it from a monitor directly.

Signed-off-by: Sébastien Han <seb@redhat.com>

roles/ceph-rgw/handlers/main.yml

@@ -1,6 +1,8 @@
 ---
 - name: update period
   command: radosgw-admin period update --commit
+  delegate_to: "{{ groups[mon_group_name][0] }}"
+  run_once: true
 
 - name: restart rgw
   service: