Add Constraints support

To be co-installable with other OpenStack projects and have consistent
testing it's a good idea (tm) to use constraints when creating tox
environments.

Change-Id: I69045d7f3d4e70c273163916f79be382b97c724a

tox.ini

@@ -9,7 +9,7 @@ skipsdist = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
 usedevelop = True
-install_command = {toxinidir}/tools/tox_install.sh {opts} {packages}
+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
 whitelist_externals = sh