cleanup second static cluster in test

hopefully this will remove the testing infra from failing on hanging to memory

src/test/java/org/elasticsearch/tribe/TribeTests.java

@@ -69,6 +69,7 @@ public class TribeTests extends ElasticsearchIntegrationTest {
         if (cluster2 != null) {
             cluster2.afterTest();
             cluster2.close();
+            cluster2 = null;
         }
     }