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: I5b435b46f45c1b542b5c755938ea257310038be6

openstack-dev/bashate,openstack-dev/bashate