Have tox use pip upgrade when installing

To synchronize behavior with other integrated projects for
consistency, tox uses pip install -U making sure any packages
already installed in a virtualenv are upgraded to what is specified
in the requirements lists.

Change-Id: Ida5e440d1bdb9f8e9031277ea53a02d2ef171438
Co-Authored-By: Monty Taylor <mordred@inaugust.com>
Merge "Have tox use pip upgrade when installing" into stable/havana