config: Remove confusing magic in path handling

1. When passed a .yaml path (or any path with a non-whitelisted extension),
   don't read .yml instead when the .yaml exists.
   It's an extremely surprising behavior.

2. If the .yaml file does not exist, it will still try alternative extensions
   in order not to break existing deployments which may rely on it, but it
   raises a warning now.

3. When given a non-existing path, show an error log, but keep parsing
   it as an empty config, in order not to break existing deployments.