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: I8fd9cf320c69bd7ef9571d4e7381716a3bd30862

openstack/puppet-monasca,openstack/puppet-monasca,openstack/puppet-monasca,openstack/puppet-monasca,openstack/puppet-monasca