Add py27-mitaka tox target

Adding a mitaka tox target, that will allow testing current
code against mitaka upper constraints.

Change-Id: Ia84ffefb820b376274714ae93caca148ff315c4b

tox.ini

@@ -24,6 +24,12 @@ commands = {toxinidir}/manage.py test muranodashboard --settings=muranodashboard
 sitepackages = False
 commands = flake8
 
+[testenv:py27-mitaka]
+install_command = pip install -chttps://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt?h=stable/mitaka {opts} {packages}
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+       http://tarballs.openstack.org/horizon/horizon-stable-mitaka.tar.gz
+
 [testenv:venv]
 # NOTE(from nova tox.ini by jaegerandi): This target does not use constraints
 # because upstream infra does not yet support it. Once that's fixed, we can