Deduplicate aggregate notification samples

This patch refactors the sample files of aggregate-add_host,
aggregate-delete, aggregate-create and aggregate-remove_host
notifications to use a common AggregatePayload sample data.

Change-Id: I50382f4d829693412bbfe47ffd08a24e69342715

doc/notification_samples/aggregate-create-end.json

@@ -1,18 +1,7 @@
 {
     "priority": "INFO",
     "payload": {
-        "nova_object.version": "1.1",
-        "nova_object.namespace": "nova",
-        "nova_object.name": "AggregatePayload",
-        "nova_object.data": {
-            "name": "my-aggregate",
-            "metadata": {
-                "availability_zone": "nova"
-            },
-            "hosts": [],
-            "id": 1,
-            "uuid": "788608ec-ebdc-45c5-bc7f-e5f24ab92c80"
-        }
+        "$ref": "common_payloads/AggregatePayload.json#"
     },
     "event_type": "aggregate.create.end",
     "publisher_id": "nova-api:fake-mini"