Mailman uses a (usually hidden) mailing list named "mailman" to
handle things like password reminders and certain other sorts of
notifications. We have one in the configuration for all the sites on
lists.openstack.org but not on lists.katacontainers.io, even though
the production server has one. Not creating this list will cause
the services to fail to start, and since we want to test restarting
them in an upcoming change, add the missing entry (it will be a
no-op in production anyway).
Change-Id: If06d9d060e40055f95c1df337eb6f32c6064a89f