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: I0968c6740ccb96d93527ba368eebe3e56e4e043f

openstack/aodh,openstack/aodh