Support nested netconf operations in netconf endpoint for config subsystem

The mapping component between netconf server and config subsystem does not really support netconf operations on nested nodes. It focuses on modifying the configuration as a whole for each module instance.

This limitation is only present in the netconf server for config subsystem, its mapping component is unable to handle it partly due to the way config subsystem generates its binding code (e.g. lists are represented as Lists<> and JMX represents them as Object[] ... it would be difficult to perform complex nested operations on an array of objects)

Netconf server for MD-SAL can already handle this ... so if we were to migrate config subsystem into using MD-SAL (at least the datastore) internally, we would get this functionality out of the box.