Enable core to default networking to Netty 4

This commit enables core to set the default transport and HTTP
implementation to use transport-netty4.

Relates elastic/elasticsearch#2999

Original commit: elastic/x-pack-elasticsearch@cb0efa29ab

elasticsearch/x-pack/security/src/main/java/org/elasticsearch/xpack/security/Security.java

@@ -333,7 +333,10 @@ public class Security implements ActionPlugin, IngestPlugin {
 
     public Settings additionalSettings() {
         if (enabled == false) {
-            return Settings.EMPTY;
+            return Settings.builder()
+                    .put(NetworkModule.HTTP_TYPE_KEY, "netty3")
+                    .put(NetworkModule.TRANSPORT_TYPE_KEY, "netty3")
+                    .build();
         }
 
         return additionalSettings(settings);