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: I9e985c4942ad3843b1c0af63d5504c690c836b26

openstack/python-ironicclient,openstack/python-ironicclient