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: I342817b1ef813a190d97c61166bc303bc198d319

tox.ini

@@ -1,11 +1,14 @@
 [tox]
+minversion = 2.0
 envlist = py27,py34,py35,pep8
 
 [testenv]
+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}/test-requirements.txt
     -r{toxinidir}/requirements.txt
 setenv =
+    VIRTUAL_ENV={envdir}
     PYTHONPATH = {toxinidir}
 commands = nosetests --tests almanach.tests.unit