Verify next-available limit timestamps are isotime

We format these timestamps using timeutils.isotime() in the limits
view builder, 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: I14bf2eccec38f271eec8d97e8dca6d46f9d93cb3
Merge "Verify next-available limit timestamps are isotime"