Update time is not updated when metadata of aggregate is updated

For example, When aggregate zone is updated, time is not getting updated
in updated_at field.

So, modified the code such that time gets updated in updated_at field
whenever the aggregate metadata content gets modified.

Change-Id: Icb65313ba85562fadeddbc1890ca5d463e74d3c2
Closes-Bug: #1538014

doc/api_samples/os-aggregates/aggregates-metadata-post-resp.json

@@ -11,6 +11,6 @@
             "key": "value"
         },
         "name": "name",
-        "updated_at": null
+        "updated_at": "2013-08-18T12:17:55.986540"
     }
 }

nova/compute/api.py

@@ -3670,6 +3670,7 @@ class AggregateAPI(base.Base):
                                   action_name=AGGREGATE_ACTION_UPDATE)
         if values:
             aggregate.update_metadata(values)
+            aggregate.updated_at = timeutils.utcnow()
         self.scheduler_client.update_aggregates(context, [aggregate])
         # If updated values include availability_zones, then the cache
         # which stored availability_zones and host need to be reset
@@ -3689,6 +3690,7 @@ class AggregateAPI(base.Base):
         # which stored availability_zones and host need to be reset
         if metadata and metadata.get('availability_zone'):
             availability_zones.reset_cache()
+        aggregate.updated_at = timeutils.utcnow()
         return aggregate
 
     @wrap_exception()

nova/tests/functional/api_sample_tests/api_samples/os-aggregates/aggregates-metadata-post-resp.json.tpl

@@ -11,6 +11,6 @@
             "key": "value"
         },
         "name": "name",
-        "updated_at": null
+        "updated_at": %(strtime)s
     }
 }