tempest tox venvs brakes CI

[Lenny]

The following commit[1] brakes Mellanox CI[2]

[1] https://review.openstack.org/#/c/311840/2
[2] http://144.76.193.39/ci-artifacts/71/312771/2/check-sriov-tempest/Tempest-Sriov/bcd2657/console.html.gz

07:22:29 2016-05-05 04:21:52.570 | ++ lib/tempest:install_tempest:613 : /opt/stack/tempest/.tox/full/bin/pip install -c /opt/stack/requirements/upper-constraints.txt -r requirements.txt
07:22:29 2016-05-05 04:21:52.571 | /jenkins/workspace/Tempest-Sriov/devstack/lib/tempest: line 613: /opt/stack/tempest/.tox/full/bin/pip: No such file or directory
07:22:29 2016-05-05 04:21:52.575 | + lib/tempest:install_tempest:1 : exit_trap

[Sean Dague (sdague)]

This devstack bug was last updated over 180 days ago, as devstack
is a fast moving project and we'd like to get the tracker down to
currently actionable bugs, this is getting marked as Invalid. If the
issue still exists, please feel free to reopen it.