Teach subunit2html to use v2 when it is available.

This changes the API of the result object to be more standard with
Python2.7+ APIs.

Change-Id: I1dd3da2280d687a591efd069ef43b1f2bc40e9f1
Reviewed-on: https://review.openstack.org/23670
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins