LP#1793585: Fix PostgreSQL apt repo source setup

LP#1793585: Fix PostgreSQL apt repo source setup

For standalone DB server install, ensure that we are applying the proper
PostgreSQL community apt repo for packages.

DO NOT backport this branch.  There are presently known issues with
Ubuntu 18.04 and email generation from action/trigger and other
interfaces.  We plan to address these before the 3.3 release, but do
not necessarily expect to backport those fixes.

Signed-off-by: Ben Shum <ben@evergreener.net>
Signed-off-by: Jason Stephenson <jason@sigio.com>