Make sure pip upgrades itself in tox

* tox.ini: Upgrade pip within tox virtualenvs for
openstack-infra/config. Without doing this, tox uses the system pip
version within the constructed virtualenv, and may inadvertently
pull in prerelease versions of dependencies from PyPI. Note that the
option used requires tox 1.6 or newer.

Change-Id: Ie5276a4437e72c5f5a8d1ecf5a7c07977fa5bd77
Merge "Make sure pip upgrades itself in tox"