Centralize NETCONF capability URNs

The URNs used to identify NETCONF capabilities are used also as XML namespaces and we have the definitions of these strewn all across the codebase.

While we are not yet ready to deal with the task of defining a complete object model of NETCONF capabilities, having a central place for their identifiers is helpful.

Create CapabilityURN utility class, which will hold the Strings of the known capability URNs. Also eliminate duplicate definitions so that users rely exclusively on these constants.