Use actual fake timestamp in API templates

The 2011-01-01T01:02:03Z is seen throughout the images API sample
tests because it is hard-coded in the fake image service. In some
of the API templates, we include the timestamp rather than matching
against a regexp but in many we don't. Let's consistently match
against the actual timestamp we know to expect.

Full context here:

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

Change-Id: I727c0aabac0e32b435068c37d535daa8c983364c

nova/tests/integrated/api_samples/OS-EXT-IMG-SIZE/image-get-resp.json.tpl

@@ -29,6 +29,6 @@
         "OS-EXT-IMG-SIZE:size": %(int)s,
         "progress": 100,
         "status": "ACTIVE",
-        "updated": "%(timestamp)s"
+        "updated": "2011-01-01T01:02:03Z"
     }
 }