FaceSystem class is the entry point of NFD's face system. It owns
ProtocolFactory objects that are created from face_system section of
NFD configuration file. This responsibility is split from FaceManager.
This commit also adds test coverage for face_system.websocket config
section processing routine.
refs #3904
Change-Id: If9a23c23b81f03d2b76c29febb3f29e6007d595b