Comment 2 for bug 1675328

It's not working because there is a function to delete the postgres superuser - https://github.com/openstack/trove/blob/stable/newton/trove/guestagent/datastore/experimental/postgresql/manager.py#L126-L134

I don't know why you decided to delete the postgres superuser, but PostgreSQL 9.3-9.5 requires this user in order to start a PostgreSQL cluster. (In the pg_ctlcluster script in Ubuntu Trusty and Xenial)