Accept then discard mails from Phabricator instances

A new postfix_config.discard_mailboxes pillar entry allows to list valid
mailboxes that act as a blackhole: mails will be accepted, then discarded.

Set DevCentral no-reply adresss as first candidate.

Ref T2349

Test Plan:

• check logs for Phabricator no-reply mails
• ensure regular virtual mailboxes are still working

Differential Revision: https://devcentral.nasqueron.org/D4190