This commit contains multiple changes:
 - Start the registration task after the ApplicationReady event. This makes
   AdminClientProperties.isReady() unnecessary.
 - The RegistarApplicationListener has two new public methods to start/stop the
   periodic registration task.
 - Add spring.boot.admin.auto-registration property (default: true). When this
   property is set the registration task is automatically scheduled after the
   ApplicationReadyEvent.

closes #151