RedfishManagement.get_mac_addresses has two problems:
* It returns a dict while the base class documents returning a list
* The dict's values are Redfish-specific values
This change fixes this function to return a list and significantly
simplifies the related create_ports_if_not_exists.
Change-Id: I329cabe04662d0d668d4c3e04ecede5b4fdec6c6