Add more logging to try and track down the root cause of ONOS-4605.

Change-Id: Ife0663ca16565b990fbae97182e97c33f0df60ae

@@ -207,6 +207,11 @@ public class DistributedNetworkConfigStore
         ImmutableSet.Builder<Class<? extends Config<S>>> builder = ImmutableSet.builder();
         configs.keySet().forEach(k -> {
             if (Objects.equals(subject, k.subject) && k.configClass != null && delegate != null) {
+                ConfigFactory<S, ? extends Config<S>> configFactory = factoriesByConfig.get(k.configClass);
+                if (configFactory == null) {
+                    log.error("Found config but no config factory: subject={}, configClass={}",
+                            subject, k.configClass);
+                }
                 builder.add(factoriesByConfig.get(k.configClass).configClass());
             }
         });