Ensure there is only one shutdown hook

The effect of not doing this is not often noticeable, but you sometimes
get a race condition between a context and its parent closing in different
threads without it.

spring-cloud-context/src/main/java/org/springframework/cloud/bootstrap/BootstrapApplicationListener.java

@@ -141,6 +141,7 @@ public class BootstrapApplicationListener implements
 			}
 		}
 		if (!installed) {
+			application.setRegisterShutdownHook(false);
 			application.addInitializers(new AncestorInitializer(context));
 		}