[bs-28] Add /shutdown endpoint to service apps

Disabled by default use container.allow_shutdown=true to switch
it on.  Then POST to /shutdown to shut down the app.

[#48059059]

spring-bootstrap/src/main/java/org/springframework/bootstrap/context/embedded/jetty/JettyEmbeddedServletContainer.java

@@ -56,7 +56,11 @@ public class JettyEmbeddedServletContainer implements EmbeddedServletContainer {
 	@Override
 	public synchronized void stop() {
 		try {
+			this.server.setGracefulShutdown(10000);
 			this.server.stop();
+		} catch (InterruptedException ex) {
+			Thread.currentThread().interrupt();
+			// No drama
 		} catch (Exception ex) {
 			throw new EmbeddedServletContainerException(
 					"Unable to stop embedded Jetty servlet container", ex);