BUG-868: do not use InstanceIdentifier.getPath()

This simplifies the NodeMapping class significantly by using the
utility methods shipped in InstanceIdentifier itself.

Change-Id: I1bb79bd4a16da9d78c600d21f67e3ca1926a0445
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-868: do not use InstanceIdentifier.getPath()"