exim: get rid of the implicit secondary MX feature

Our exim secondary relay(s) right now accept (and relay) emails under
two conditions: 1) by explicitly checking against the relay_domains
domainlist, i.e. our configured-in-the-primary domains
(wikimedia_domains); 2) by implicitly accepting all mail for which MX
records point to our primary/secondary couple.

Our primary MXes have a similar behavior, by matching the lowest
priority MX record with their hostname and accepting emails regardless
of configuration.

Because of our primary localpart/alias config, this means that we accept
emails, just for postmaster@ for a number of unconfigured domains.

This lazy/implicit acceptance of emails is a bit evil, as it means that
anyone can set up MX records on their domain and spam postmaster@. It
also results in undocumented behavior as we have no record of these
domains, which could be especially interesting with domains not under
our control, such as wikilovesmonuments.org.

Remove this implicit handling and rely only on our canonical domain
list. This affects just the postmaster@ address of 124 domains which
shouldn't have had email handling in the first place.

Change-Id: I23bb88c2716db014e8d16e5d4b7ebfbdfbd05722
mail: add a root system alias to role::mail::mx

On the regular non-MTA boxes, our minimal config has unknown aliases
aliased implicitly to root@wikimedia.org. root is not defined by default
on Ubuntu systems, so it gets caught in the catch-all.

MTA boxes, such as role::mail:: ones, don't have that kind of exim
config and arguably, they shouldn't. Add a system mail alias instead, to
alias root (and hence postmaster etc.) to root@wikimedia.org.

Change-Id: I437f603d5ff8f05fd094af086a86e674dcc8be12