User from databases.json should have higher priority over user passed from pgb-notify

For example, in databases.json if we have

{"database":{"host":"127.0.0.1","port":"5431","user":"pgbouncer","password":"hunter1"}}

and node['gitlab']['pgbouncer']['databases_ini_user'] is gitlab-psql, we will end up in databases.ini with:

[databases]

database = host=127.0.0.1 port=5431 auth_user=gitlab-psql

Setting databases_ini_user to pgbouncer won't work because usually there's no system user for such user (which is needed by omnibus)