Be able to configure whether the connector uses device models or standards one for netconf base operations

Be able to make the netconf connector configurable (whether the connector should use device models for netconf base operations or a standard one).

Add the functionality for the netconf connector to ignore any schemas touching base netconf operations and use pre-loaded models that are already in ODL.

The purpose is to have an even more open netconf connector, so it can be compatible with any netconf server, for instance ConfD with all its tail-f extensions.