Changed minversion in tox to 3.18.0
Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Id8bdda703afc39d352e3a53877318dc30d91a5f7

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,nawawi/openstack