Update minimum tox version to 2.3.2
Update minimum tox version to 2.3.2

After latest changes on tox.ini we need to change minimum
version of tox to 2.3.2. Problem is described in tox changelog
as #issue279. Without this change tests are unable to run.

Change-Id: I458cf0d5e8927cf11a44b49ecfb5178a228aafaf
Closes-Bug: #1695888

mahak/neutron,huntxu/neutron,openstack/neutron,noironetworks/neutron,huntxu/neutron,openstack/neutron,mahak/neutron,eayunstack/neutron,eayunstack/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron