Tests: Add back tests.cluster

This setting is used by the release script to run rest tests against
the version being released.  It used to work only for tests using
the global cluster.  Now it supercedes both SUITE and TEST scope
test clusters.

closes elastic#9916

src/test/java/org/elasticsearch/test/ElasticsearchIntegrationTest.java

@@ -1032,8 +1032,8 @@ public boolean apply(Object o) {
      */
     public void setMinimumMasterNodes(int n) {
         assertTrue(client().admin().cluster().prepareUpdateSettings().setTransientSettings(
-                settingsBuilder().put(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES, n))
-                .get().isAcknowledged());
+            settingsBuilder().put(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES, n))
+            .get().isAcknowledged());
     }
 
     /**
@@ -1593,8 +1593,34 @@ protected Settings nodeSettings(int nodeOrdinal) {
     protected Settings transportClientSettings() {
         return ImmutableSettings.EMPTY;
     }
+
+    private ExternalTestCluster buildExternalCluster(String clusterAddresses) {
+        String[] stringAddresses = clusterAddresses.split(",");
+        TransportAddress[] transportAddresses = new TransportAddress[stringAddresses.length];
+        int i = 0;
+        for (String stringAddress : stringAddresses) {
+            String[] split = stringAddress.split(":");
+            if (split.length < 2) {
+                throw new IllegalArgumentException("address [" + clusterAddresses + "] not valid");
+            }
+            try {
+                transportAddresses[i++] = new InetSocketTransportAddress(split[0], Integer.valueOf(split[1]));
+            } catch (NumberFormatException e) {
+                throw new IllegalArgumentException("port is not valid, expected number but was [" + split[1] + "]");
+            }
+        }
+        return new ExternalTestCluster(transportAddresses);
+    }
 
     protected TestCluster buildTestCluster(Scope scope, long seed) throws IOException {
+        String clusterAddresses = System.getProperty(TESTS_CLUSTER);
+        if (Strings.hasLength(clusterAddresses)) {
+            if (scope == Scope.TEST) {
+                throw new IllegalArgumentException("Cannot run TEST scope test with " + TESTS_CLUSTER);
+            }
+            return buildExternalCluster(clusterAddresses);
+        }
+
         final String nodePrefix;
         switch (scope) {
             case TEST: