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: Ie74cd61e251d46524d83a05c0b48fd462aeb4a57

rahulunair/kolla,openstack/kolla,stackforge/kolla,stackforge/kolla,stackforge/kolla,openstack/kolla,rahulunair/kolla