realm: rename exim_default_route_list & exim_mediawiki_route_list

Rename them to $mail_smarthost and $wikimail_smarthost respectively, as
they're not really an exim concept. While at it, also convert them into
proper puppet arrays, so that they can be used all over the puppet tree
as well.

Change-Id: I4b94507efa785fd6dcea8b54a1d441b1731f3fa0