Change tox file

Add upper-constraints and min tox version.

This will prevent taking in packages that
are greater then what is defined in the
upper-constraints

Change-Id: I4a7aea0900efa767a4a557d20d601caa9938dfeb

tox.ini

@@ -1,11 +1,13 @@
 [tox]
 envlist = py27,pypy,pep8
+minversion = 2.0
 skipsdist = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command =
+    pip install -U -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
 whitelist_externals = find