Use datetime object for stub created_at timestamp

The stubbed out volumes methods returns a volume object with a
pre-serialized datetime object which doesn't give us the opportunity
to test the serialization code properly. Use a real object instead.

Full context here:

  http://lists.openstack.org/pipermail/openstack-dev/2014-April/033971.html

Change-Id: Ie54a3deaf1c32f3e57d03737e9ea1e9dd66a98d8

nova/tests/integrated/test_api_samples.py

@@ -3986,7 +3986,7 @@ class VolumesSampleJsonTest(ServersSampleBase):
                   'name': 'vol name',
                   'display_name': displayname,
                   'display_description': displaydesc,
-                  'created_at': "2008-12-01T11:01:55",
+                  'created_at': datetime.datetime(2008, 12, 1, 11, 1, 55),
                   'snapshot_id': None,
                   'volume_type_id': 'fakevoltype',
                   'volume_metadata': [],