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: I84a53543b50184b4e838cd6e597dfc80ec025588

openstack/sahara-image-elements,openstack/sahara-image-elements