phabricator: Fix mailer settings
phabricator: Fix mailer settings

Phabricator has refactored it's config. The "metamta.mail-adapter"
setting has been replaced by "cluster.mailers" with a different
structure.

Change-Id: I328cb182282bc80839acd6529834cdaca3c91248

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant