use port-element-id over portno

At the moment the portno of the physical_adapter_mappings defaults to 0 when none is given. The idea behind that was, that it does not need to be specified on single port adapters. But what happens if an adapter with the port element id "foo" gets introduced (which is not 0). In this case the current logic will fail. To work around the whole problematic the option example should get renamed to

<portno> -> <port-element-id> (if still used somewhere)
To be discussed: <port-element-id> should become mandatory?