GHSA-fgmc-2hqj-86v4: Vantage6: Set admin user and password from environment or configuration

Vantage6 currently provides an initial user with username root and password root. This is not ideal for the following reasons:

• Attackers know that almost all vantage6 servers have a user with username root that probably has admin rights
• The initial password is very weak and it is possible that administrators forget to reset it.