Remove global python-subunit install.

The global python-subunit install that was used for subunit log
processing is interfering with the version desired for nova because nova
enabled sitepackages.

Remove the global install and use the local virtualenv install of
python-subunit to process subunit logs. This ensures compatibility with
testrepository and the subunit logs themselves.

Change-Id: Id634b7f96bcdb594242c0ae5734f77a9fdf5425c
Reviewed-on: https://review.openstack.org/26396
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins