remove default finalizer name

Author: Soroosh Sarabadani

operator-framework/src/main/java/io/javaoperatorsdk/operator/api/Controller.java

@@ -10,14 +10,17 @@
 @Retention(RetentionPolicy.RUNTIME)
 @Target({ElementType.TYPE})
 public @interface Controller {
-
-    String DEFAULT_FINALIZER = "operator.default.finalizer";
+    String NULL = "";
 
     String crdName();
 
     Class<? extends CustomResource> customResourceClass();
 
-    String finalizerName();
+    /**
+     * Optional finalizer name, if it is not,
+     * the canonical name of the Controller class will be used as the name of the finalizer.
+     */
+    String finalizerName() default NULL;
 
     /**
      * If true, will dispatch new event to the controller if generation increased since the last processing, otherwise will