[Rest] Add the cluster name to the "/" endpoint

The root endpoint returns basic information about this node, like it's name and ES version etc. The cluster name is an important information that belongs in that list.

Closes #7524

rest-api-spec/test/info/10_info.yaml

@@ -3,6 +3,7 @@
     - do:         {info: {}}
     - match:      {status: 200}
     - is_true:    name
+    - is_true:    cluster_name
     - is_true:    tagline
     - is_true:    version
     - is_true:    version.number

src/main/java/org/elasticsearch/rest/action/main/RestMainAction.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.action.admin.cluster.state.ClusterStateRequest;
 import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
 import org.elasticsearch.client.Client;
+import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
@@ -41,11 +42,13 @@
 public class RestMainAction extends BaseRestHandler {
 
     private final Version version;
+    private final ClusterName clusterName;
 
     @Inject
-    public RestMainAction(Settings settings, Version version, Client client, RestController controller) {
+    public RestMainAction(Settings settings, Version version, Client client, RestController controller, ClusterName clusterName) {
         super(settings, client);
         this.version = version;
+        this.clusterName = clusterName;
         controller.registerHandler(GET, "/", this);
         controller.registerHandler(HEAD, "/", this);
     }
@@ -80,6 +83,7 @@ public RestResponse buildResponse(ClusterStateResponse response) throws Exceptio
                 if (settings.get("name") != null) {
                     builder.field("name", settings.get("name"));
                 }
+                builder.field("cluster_name", clusterName.value());
                 builder.startObject("version")
                         .field("number", version.number())
                         .field("build_hash", Build.CURRENT.hash())