Blackhole deliveries for Mailman v3 local user

On Mailman v2 the "mailman" addresses were mapped to special mailing
lists used for monthly password notifications and some other tasks.
This does not exist on Mailman v3, but spammers still have the old
mailman list addresses and send junk to them, which the server
attempts to deliver because there's a local user account with the
same name.

Reject messages for the old "mailman" addresses at receipt, so they
never enter our message queue.

Change-Id: I9db93ae98f4b3952400c1e478612ab70a6241dd1

inventory/service/host_vars/lists01.opendev.org.yaml

@@ -31,6 +31,9 @@ exim_aliases:
   product-wg-owner: spam
   user-committee-owner: spam
   spam: ':fail: delivery temporarily disabled due to ongoing spam flood'
+  # This is the local username for mailman processes, but it does not send nor
+  # need to receive messages.
+  mailman: ':blackhole: this address does not accept email'
   # TODO It would be better to bypass verification for postorius@listdomain
   # and set a :fail: rule for anyone trying to send email to this addr.
   # But that requires updating our main exim config so that needs more thought.

playbooks/zuul/files/host_vars/lists99.opendev.org.yaml

@@ -40,6 +40,9 @@ exim_aliases:
   product-wg-owner: spam
   user-committee-owner: spam
   spam: ':fail: delivery temporarily disabled due to ongoing spam flood'
+  # This is the local username for mailman processes, but it does not send nor
+  # need to receive messages.
+  mailman: ':blackhole: this address does not accept email'
   # TODO It would be better to bypass verification for postorius@listdomain
   # and set a :fail: rule for anyone trying to send email to this addr.
   # But that requires updating our main exim config so that needs more thought.