pgbouncer: Run pgbouncer as postgres user

First things first, iso production deployment.

At the moment, the pgbouncer module wants (reasonably) a dedicated pgbouncer
user to run it (and that fails as this user does not exist in our manifest).

I'm currently modifying this default setup to run as current production
runs (postgres user runs it).

This can be adapted later if everything is fine and that needs to be changed.

Related T1784