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: Ic53cd023d9007321441bafd6c8e4c9b4f39e58b3

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command  = pip install -U {opts} {packages}
+install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 setenv =
     PYTHONUNBUFFERED=1
     VIRTUAL_ENV={envdir}