Verify created_at cloudpipe timestamp is isotime

We format these timestamps using timeutils.isotime() in the cloudpipe
API extension, so we should check that in the API sample tests rather
than using the generic timestamp regexp.

Full context here:

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

Change-Id: I52543f68a512da4064a905e2efa453425ce36dae
Merge "Verify created_at cloudpipe timestamp is isotime"