Update the min tox version to 2.0
Update the min tox version to 2.0

It is always safe to define the minimal tox version required to run,
if the host tox is less than this the tool will exit with an error
message indicating the user needs to upgrade tox.

For ref: https://tox.readthedocs.io/en/latest/config.html#tox-global-settings

Change-Id: I3b387930451d78e20a69b8c29b0b28290673ef78

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates