This changes adds logging message:debug and error. fmt.Errorf() is
wrapped by logging.Errorf() hence all error message is also put to
log file. logging.Debugf() is called at almost function call, so
we could track the code by logging message.
Plugin went away, replaced by a minimal Spec.Config field so that the CNI
version the thick plugin supports could be indicated. This means we need
to also inject the network name into the minimal CNI JSON config when
loading the Spec.Config if the network name isn't there already.