Use datetime object for fake network timestamps

The fake networks use a pre-serialized timestamp in str(datetime)
format which which doesn't give us the opportunity to test the
serialization code properly and isn't the format use in practice.

Use real dateime objects instead.

Full context here:

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

Change-Id: I5cbe722fb59cdd326b4c75dcfea14f206d71f668
Merge "Use datetime object for fake network timestamps"