Move "mail aliases to OIT" cron to role::mail::mx

As mchenry (oldmx) is going away soon, move the cronjob to the main MX,
doubling OIT's cronspam in the process.

While at it, fix an issue where the cron was depending on the "apache"
user, and inline the misc::maintenance::mail_exim_aliases class into the
role class for simplicity.

Change-Id: I891c221a6301568471482e8af46b36d39248d858