Feature/472 convert config to yml

Converts config to Yml, validates it and makes config easier to parse.
Yes and no...
Yes: if you would directly get a specific metadata configuration, it probably will not find it.
No: This is not done currently and the config proxy is a workaround which should be remove anyway.  Furthermore If you ask me the metadatafields config is the first candidate for being 'migrated' If you ask me. Instead of parsing the current config to something usable (for example to feed to a form) in runtime this could be done just once.