replace whitelist_externals by allowlist_externals
replace whitelist_externals by allowlist_externals

replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Depends-on: https://review.opendev.org/c/openstack/designate/+/796597
Change-Id: Id28a67abc74c3e14d4cba8d3278c89a1fe029252

openstack/designate,openstack/designate,openstack/designate