Preupgrade setuptools in tox tempestenv

* tox.ini: If setuptools is declared as a dependency, even
transitively, it can trigger a race in some versions of pip when
called with -U wherein pip removes setuptools in preparation for
upgrading it, and then proceeds to install something else using
setuptools. Work around this behavior by upgrading setuptools within
the virtualenv in advance.

Change-Id: I6d9bf61c1ce41e46ff6f621bfa88d511f6c16120
Related-Bug: #1405579
Merge branch 'master' of https://github.com/openstack/tempest