WiP: update Postfix configuration with one that we know works

This has a sub-optimal aspect in that local_recipient_maps being
set to empty can cause "backscatter", as described here:

  http://www.postfix.org/postconf.5.html#local_recipient_maps

We should set that to only allow the configured domain instead.
Only set up /etc/rc.local on EC2 servers with --default