Fix group read permissions

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.

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

Change-Id: I5e680e7546ad66f087c1c15cd2db60428d50afb9
Closes-Bug: 1782644

src/lib/charm/openstack/designate.py

@@ -346,6 +346,8 @@ class DesignateCharm(openstack_charm.HAOpenStackCharm):
         ]),
     }
 
+    group = 'designate'
+
     def __init__(self, release=None, **kwargs):
         """Custom initialiser for class
         If no release is passed, then the charm determines the release from the