Allow separate configuration files per environment and runtime

[Taucher2003]

Instead of reading only sagittarius.yml we should also read environment and runtime specific configuration files.

That way configuration options can be set in the following files:

• sagittarius.yml
• sagittarius.[environment].yml
• sagittarius.[runtime].yml

Valid environment options are production, development and test (basically Rails.env)
Valid runtime options are puma, good_job and grpc (maybe also rake and console)

All relevant files are deep merged with runtime specific file having the highest precedence, then environment and lastly the generic sagittarius.yml.