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: I400b848c79955a8fcb08cdd46e043660df67589c

tox.ini

@@ -4,7 +4,7 @@ distribute = False
 envlist = py35,py27,pep8
 
 [testenv]
-install_command = pip install -U {opts} {packages}
+install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/test-requirements.txt