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

The verification comands used by tox need at least 2.0 version. This
patch aims to update tox version from 1.6 to 2.0 which should help with
local running of constraints targets.

Change-Id: Ida5673ee66ab320975ead2d518df4f27c07b6d77

openstack/reno