API change, we need to be able to store the SF tap port in the DPL for OPNFV SFC

In the context of OPNFV SFC, we need to be able to configure the tap port the SF is connected to in the SF DPL configuration. The VNF Mgr (Tacker) will know this info and can easily configure it. Currently Tacker is putting the tap port name in the SF DPL name, but there should be a dedicated field for it.

I propose we augment the SF Yang model and add it.

Brady