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: I02478c2d39f1ce571e97e6187db0d4e0dfde8f70

tox.ini

@@ -4,7 +4,7 @@ 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=VIRTUAL_ENV={envdir}
 deps=-r{toxinidir}/requirements.txt
      -r{toxinidir}/test-requirements.txt