Add -U to the command line for pip

Since we have site-packages=True in nova, the existence of system
packages of the wrong version can cause nova to not install what
you expect into the tox virtualenv. Adding -U into the deps list
causes it to get appended into the command line, which causes pip
to do what we want and install what we asked for.

Change-Id: I4f388e07caf31ab870f22d261e856a205ebb36c0

tox.ini

@@ -7,7 +7,8 @@ setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C
-deps = -r{toxinidir}/requirements.txt
+deps = -U
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   python tools/patch_tox_venv.py