Vantage6: Set admin user and password from environment or configuration

vantage6 is an open-source infrastructure for privacy preserving analysis. Versions prior to 5.0.0 provide an initial user with username `root` and password `root`. This is not ideal because attackers know that almost all vantage6 servers have a user with username `root` that probably has admin rights, and the initial password is very weak and it is possible that administrators forget to reset it. Version 5.0.0 fixes the issue. As a workaround, it is possible to delete the `root` user after it has been used to create other users.