Test: Don't force networking mode.

By default use local transport to increase testing speed.
Also Zen discovery is required for plugin tests otherwise Guice errors occur (so far unclear to me what is causing this).

Original commit: elastic/x-pack-elasticsearch@0de574c12f

src/test/java/org/elasticsearch/alerts/AbstractAlertingTests.java

@@ -53,7 +53,7 @@ public abstract class AbstractAlertingTests extends ElasticsearchIntegrationTest
                 .put(super.nodeSettings(nodeOrdinal))
                 .put("scroll.size", randomIntBetween(1, 100))
                 .put("plugin.types", AlertsPlugin.class.getName())
-                .put("node.mode", "network")
+                .put("discovery.type", "zen")
                 .build();
     }