Remove -U from pip install

'pip install -U' ugrades specified packages, this is not necessary
since we use constraints, remove the parameter '-U' from the line.

With tools/tox_install.sh - which a previous change of mine removed -
the -U was not harmful, but with the current set up, it might cause
upgrades, so remove it.

Change-Id: Ia27e31299490771dcf3460bf5581ef50dfea7b34

tox.ini

@@ -9,7 +9,7 @@ deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-install_command = pip install -U {opts} {packages}
+install_command = pip install {opts} {packages}
 commands =
   python setup.py testr --slowest --testr-args='{posargs}'