Use RFC 8040 restconf implementation in netconf CSIT

On a long term basis, we should aim for removal of bierman-draft-02 restconf implementation and use just RFC 8040 restconf implementation. Benefits are obvious, we do not have to maintain and support two implementations and we want to be RFC compliant, not draft compliant.

The first step on this journey is that we should have at least some "user" of new restconf. So we should duplicate the netconf CSIT tests, one will talk to bierman one will talk to RFC. This way we can say that at least some basic behavior is supported by RFC 8040 implementation, so other CSIT can migrate to use it.