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/common_payloads/AggregatePayload.json

@@ -0,0 +1,14 @@
+{
+    "nova_object.version": "1.1",
+    "nova_object.namespace": "nova",
+    "nova_object.name": "AggregatePayload",
+    "nova_object.data": {
+        "name": "my-aggregate",
+        "metadata": {
+            "availability_zone": "nova"
+        },
+        "uuid": "788608ec-ebdc-45c5-bc7f-e5f24ab92c80",
+        "hosts": [],
+        "id": 1
+    }
+}