Upgrade tox to 2.0
Upgrade tox to 2.0

Several projects have set the minimum tox version to 2.0.
This patch updates tox.ini to set the minimum required tox version
to 2.0

Change-Id: Id88e63d953d99aeebfd909ce3fae6295ddb461e1

openstack/os-brick,openstack/os-brick