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: Ie4c122e5fbb77f6e03e384a58745c1a46d91acb2

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates