add multiple super users

[NyakudyaA]

Allow env POSTGRES_USER and POSTGRES_PASS to pass multiple users and passwords because in production env we always have more than one superuser

TODO

• [ ] Add test
• [ ] Fix how secrets are affected by here

[NyakudyaA]

@tharanathkartoza This might help in GeoNode init, where we can skip having an init script