Root `polaris-server.yml` not usable

[snazy]

Describe the bug

The polaris-server.yml in the root folder is made for testing purposes, and is by far not usable in production-ish setups. This is not nice and causes a lot of confusion and try-and-error round-trips - most of those errors aren't immediately visible to end users and only discoverable using an attached debugger.

Summary of the issues:

• The default-realm cannot be bootstrapped - something like export POLARIS_BOOTSTRAP_DEFAULT-REALM_ROOT_CLIENT_ID=default_root_client, as documented, does not work in e.g. bash - also: nothing says that the realm and principal name must be upper case in the env var names.
• oauth2 is configured to test - that doesn't make sense for prod use cases
• authenticator same as for oauth2
• tokenBroker is not configured and not mentioned at all in the config file
• defaultRealm and defaultRealms options are at least confusing

The code design makes it extremely hard for users, even engineers, to figure out that and even harder why things do not work.

I propose to use this issue to come up with a better config mechanism via #469 and follow-ups.

The default configuration should at least work.

[snazy]

Closing after #469