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: I9b4c39a0d33fabadb1595389281466ba5d2453b2

stackforge/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,dtroyer/python-openstacksdk,openstack/python-openstacksdk,openstack/python-openstacksdk