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: I83d684d90309ad6109e7f6c21a9c2cad4a5371e2

tox.ini

@@ -7,7 +7,7 @@ envlist = py35,py34,py27,pep8,pypy
 usedevelop=True
 whitelist_externals = find
                       rm
-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}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY