Let's bump minimal tox version so that we can rename
whitelist_externals option to allowlist_externals one.
https: //tox.wiki/en/latest/changelog.html#v3-18-0-2020-07-23
Change-Id: I4b79e36b5ee3aefc59d262abe443271ad0dec100
devstack-plugins-nfs does not have the doc infra set
up and no doc publish job.
This is need to add the contributor doc as per the ussuri
community goal.
Change-Id: Ie091ebd4dc35ef429d60c41d74a9344b3f083718