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 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: Ifb8d4f589c2fb8fa2adc0372d1d54077afe5b32d

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar