stop delivering non-existing local user mail to leap-mx (#5431)

When mail comes in to the system, a lookup is done to see if it is a valid leap user, if it is, leap_mx now returns something of the form: uuid@deliver.local (see #5959). The virtual_mailbox_domains lists deliver.local, so postfix choses to deliver to virtual_mailbox_base (/var/mail/vmail) which has been hardcoded to the 'vmail' maildir and user.

We want leap related mail and leap aliases to go through the virtual alias system, all the hard-coded universal aliases we want to go through the local system and we dont want these separate. Known domains that are considered 'virtual' will be forwarded or delivered to the vmail user, the rest rejected as unknown recipient, instead of being handed off to leap-mx.

Previously, the way this was done is we leaned (too heavily) on the 'luser_relay' postfix configuration which sent anything that wasn't locally configured right to the leap_mx spool. That meant everything went there, including addresses that didn't exist, and leap-mx would then have to process those and bounce them. This removes the 'luser_relay' option, so any address that doesn't resolve properly to either a local address/alias, or a leap address or alias (through tcp lookups on 2424 and 4242) will get bounced as an unknown user.

Change-Id: I3c22e9383861b3794dd9adfd7aa6a0cf0a773a18