Use upper-constraints in tox tests

Change-Id: Ib2ef3b3d0d0733842abad97898114c3dbdab3b7a

tox.ini

@@ -6,6 +6,7 @@ skipsdist = True
 [testenv]
 whitelist_externals = bash
 setenv = VIRTUAL_ENV={envdir}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = bash -c 'if [ ! -d ./.testrepository ] ; then testr init ; fi'