Config file usability

For now the nabu configuration file is very basic. All the fields have to be filled, otherwise the validator will crash.

To do:

• Rename/delete some keys
• More default values
• Optional keys: some keys can be hidden from the configuration file.
• Re-work validator so that the possible values can be known in another module
• Future-proofing: add a mapper between "old keys" and "new key" names