bump the min tox version to 2.3.1
bump the min tox version to 2.3.1

When tox 2.3.0 uses the tox.ini provided by neutron,
the following bug is hitten :
https://bitbucket.org/hpk42/tox/issues/181
We need to set the min version of tox to 2.3.1.

Change-Id: If2c3f4f534ff23a1f649cebea15e945563173544
Closes-bug: #1535725

openstack/neutron,sebrandon1/neutron,igor-toga/local-snat,klmitch/neutron,bigswitch/neutron,MaximNevrov/neutron,igor-toga/local-snat,eayunstack/neutron,huntxu/neutron,sebrandon1/neutron,openstack/neutron,MaximNevrov/neutron,mahak/neutron,noironetworks/neutron,dims/neutron,wolverineav/neutron,bigswitch/neutron,mahak/neutron,mahak/neutron,cloudbase/neutron,openstack/neutron,noironetworks/neutron,cloudbase/neutron,wolverineav/neutron,huntxu/neutron,eayunstack/neutron,dims/neutron,klmitch/neutron