Redesign node config entries with proxy returns

- Redesign entries with proxies return values:
  - Replace _get_value and _get_unset_value with:
    - _create_value_proxy
    - _create_unset_proxy
- All entries returns proxies, which supports methods:
  - is_set
  - clear
- Remove UNSET constant (Due to above change, UNSET was no longer needed)
- Change is_set meaning for lists (empty list is always unset, because there are no items set)
- Replace += with append and extend
- Test new entries interface (with additional tests for list, because of many implementation differences)