Improve creation of postgres user

[openp2pdesign]

Similar to #7 the same issue can be found in create_user.py (here), this could be improved:

DataVisPlanner-airflow-webserver | Error with adding the default admin user, user probably already existing.
DataVisPlanner-airflow-worker | Error with adding the default admin user, user probably already existing.
DataVisPlanner-airflow-scheduler | Error with adding the default admin user, user probably already existing.