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
Merge "Use datetime object for stub created_at timestamp"