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: I868a0618fd8d2c9632d6bcaf2cee51ab45618532

openstack/solum,stackforge/solum,openstack/solum,stackforge/solum