[TEST] UnicastBackwardsCompatibilityTest should not copy internal node settings to external nodes

Recent test failures triggered by #7289 were caused by this, simply because internal node settings (transport type key) that are not supported by the external older nodes were copied to them by mistake.

src/test/java/org/elasticsearch/bwcompat/UnicastBackwardsCompatibilityTest.java

@@ -45,7 +45,7 @@ public class UnicastBackwardsCompatibilityTest extends ElasticsearchBackwardsCom
                 .put("transport.tcp.port", 9390 + nodeOrdinal)
                 .put("discovery.zen.ping.multicast.enabled", false)
                 .put("discovery.zen.ping.unicast.hosts", "localhost:9380,localhost:9381,localhost:9390,localhost:9391")
-                .put(super.nodeSettings(nodeOrdinal))
+                .put(super.externalNodeSettings(nodeOrdinal))
                 .build();
     }