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: I2a71f220ebbc969dc30b55804cb393c0e2f60605

stackforge/ospurge,openstack/ospurge,openstack/ospurge