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: Ide9a6337f26548fcac5e2e1bfd34b5934cafd456

openstack/puppet-trove,openstack/puppet-trove,openstack/puppet-trove