Rationalize sapi/dapi configuration

To be able to configure correctly sapi and dapi fields during the OTU and ODU interface creation, it would be more convenient to use the AEndApiInfo and ZEndApiInfo containers available at the otn-service-path level, and generalizing that usage also for OTU creation at the service-path level.
Such containers may be filled-in thanks to information contained in the path-description that actually contains the two ends of the service.