exim: sign with DKIM on the mail routers

Since smarthosts & relays are on the same box right now, DKIM signing
needs to be configured in a bit of a convoluted way, as to not sign
passing-through emails. Use an ACL variable, use in the dnslookup router
and split off transports for this.

Change-Id: Ifc9854607cbd046cd88795ba04c2747dfd42c267
mail: move wiki-mail-eqiad IP stanzas to site.pp

Move the interface::ip stanzas from role::mail::mx to site.pp, as the
config is IP-agnostic now and hence the role class can be reusable for
when we'll have multiple relays.

Change-Id: Ic1ff255a3d70e684fb42f948174601e8afc73403
exim: add all of our domains to wikimedia_domains

Add all of our project domains to wikimedia_domains, which in turn ends
up in the local_domains (or relay_domains) in the exim config. This list
is essentially the main Wikimedia projects, plus a few more
special-cases for which we have aliases.

Change-Id: Ic6fc0c19a94d890a7b704ec0d157eae619aed33c