Changed update_service_hierarchy function to use defaultdict

inventory_provider/storage/external_inventory.py

@@ -36,16 +36,11 @@ def update_interfaces_to_services(services):
 
 def update_service_hierarchy(records):
     r = db.get_redis()
-    children_to_parents = {}
-    parents_to_children = {}
+    children_to_parents = defaultdict(list)
+    parents_to_children = defaultdict(list)
     for relation in records:
         parent_id = relation["parent_circuit_id"]
         child_id = relation["child_circuit_id"]
-        if child_id not in children_to_parents:
-            children_to_parents[child_id] = []
-        if parent_id not in parents_to_children:
-            parents_to_children[parent_id] = []
-
         parents_to_children[parent_id].append(relation)
         children_to_parents[child_id].append(relation)