Update min tox version to 2.0
Update min tox version to 2.0

The commands used by constraints need at least tox 2.0.  Update to
reflect reality, which should help with local running of constraints
targets.

Change-Id: I1ce92db1801966b448ca0eeffb312a67ec1ea91a

openstack/rally,openstack/rally,openstack/rally,openstack/rally