Landscape scripts have hard coded postgres superuser

[Marcelo Henrique Neppel]

Landscape has some script that have hard code the postgres superuser (the default in PostgreSQL installations).

Some examples:
https://bazaar.launchpad.net/~landscape/landscape/trunk/view/head:/canonical/landscape/schema/utils.py#L85
https://bazaar.launchpad.net/~landscape/landscape/trunk/view/head:/canonical/landscape/scripts/standalone/plugins/database.py#L331

This is fine for most of the PostgreSQL installations, but if the PostgreSQL cluster is bootstrapped with another superuser name, Landscape fails when trying to bootstrap the database on PostgreSQL, because the postgres user does not exist.

One possible solution would be to have a configurable username that would replace those hard coded values in the scripts.

About the use case where we faced this issue: we are using Patroni (https://github.com/zalando/patroni) do manage a PostgreSQL deployment in the new PostgreSQL VM charm and we have some alignments to use specific superusers inside the database, like one named operator, and remove other users. For now, we are keeping the postgres superuser because, otherwise, the charm will not work with Landscape Server charm or Landscape Scalable bundle.