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: Id5ac796b49bd9dc163fb017f43aadfde515cddbe

tox.ini

@@ -1,11 +1,13 @@
 [tox]
 envlist = {py27,pypy}-{mysql,postgres},pep8
+minversion = 2.0
 skipsdist = True
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-install_command = pip install -U {opts} {packages} --pre
+install_command =
+    pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages} --pre
 whitelist_externals = find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt