Change group from root to barbican on config files

Recent changes in charms.openstack write configuration files with perms
0640; however without a group attribute on the client charm class, these
will root.root owned and this prevents the barbican-worker service from
starting.

Ensure that config files are written using root.barbican for this charm.

Change-Id: Iedce27a14b4a1adee30825274832ad3a3b606634

src/lib/charm/openstack/barbican.py

@@ -157,6 +157,8 @@ class BarbicanCharm(charms_openstack.charm.HAOpenStackCharm):
         ]),
     }
 
+    group = "barbican"
+
     def get_amqp_credentials(self):
         """Provide the default amqp username and vhost as a tuple.