Add ceph mgr capabilities

Without this change, manila's cephfs driver results in
manilaclient.common.apiclient.exceptions.Forbidden:
Access was denied to this resource. (HTTP 403)
when running rados commands.

http://docs.ceph.com/en/latest/mgr/administrator/#client-authentication

Closes-Bug: #1952763
Closes-Bug: #1962204

Change-Id: I6ad5047fff2f3109a9efe1f03a786ce0412b4161
(cherry picked from commit 496bfd569b)

src/lib/charm/openstack/manila_ganesha.py

@@ -50,6 +50,7 @@ GANESHA_CONF = '/etc/ganesha/ganesha.conf'
 
 CEPH_CAPABILITIES = [
     "mds", "allow *",
+    "mgr", "allow *",
     "osd", "allow rw",
     "mon", "allow r, "
     "allow command \"auth del\", "