Add a mechanism to elect a single leader in an ONOS cluster (ONOS-977).

Note: For now, leadership change callbacks are intentionally not implemented,
to discourage from liberally using this feature.
In the future we might have to add the callback support, but only if really
needed.

Change-Id: Ia70045aee737fbd67ec445ea7065476e3f4838aa
diff --git a/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java b/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
index 69e7b3e..422949a 100755
--- a/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
+++ b/src/main/java/net/onrc/onos/registry/controller/StandaloneRegistry.java
@@ -69,6 +69,11 @@
 	}
 
 	@Override
+	public boolean isClusterLeader() {
+	    return true;
+	}
+
+	@Override
 	public String getControllerId() {
 		return controllerId;
 	}