Redesign node config (and its entries) to remove proxies

- Simplify entries definitions in config. Now are defined only once. It
  helps also with no code completion problem in PyCharm.
- Remove few no longer needed methods from config.
- Remove few no longer needed tests (for example checking removed is_set
  entries method).
- Remove entry is_set method. Now to check if value is not set user has to
  check if is None (for single value) or empty list (for list entry).
- Remove entry clear method. Now to clear entry user has to assign None
  (for single value) or empty list (for list entry).
- Remove WitnessList, because it requires proxies.
- Replace AttributeError with KeyError during access to wrong config entry.
- Add tests for two config comparison.