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: I67e9387d942445a64d8e6dca85919f46b79295ab

openstack/puppet-vswitch,openstack/puppet-vswitch