Gate initial update of relations on having reached expected scale

At present the Keystone charm frequently initiates updates to its
relations before it has reached a stable state.

Make use information from ``juju goal-state`` to predict scale and gate
initial update of relations on having reached expected scale.

Depends-On: https://github.com/juju/charm-helpers/pull/226
Change-Id: I96d4aff7c4ec9fb9ea160c7e294581bab3103df8

hooks/keystone_hooks.py

@@ -104,6 +104,7 @@ from keystone_utils import (
     send_notifications,
     is_db_ready,
     is_db_initialised,
+    is_expected_scale,
     filter_null,
     is_service_present,
     delete_service_entry,
@@ -310,6 +311,10 @@ def update_all_identity_relation_units(check_db_ready=True):
         log("Database not yet initialised - deferring identity-relation "
             "updates", level=INFO)
         return
+    if not is_expected_scale():
+        log("Keystone charm and it's dependencies not yet at expected scale "
+            "- deferring identity-relation updates", level=INFO)
+        return
 
     log('Firing identity_changed hook for all related services.')
     for rid in relation_ids('identity-service'):