Better handling of anonymously accessible endpoints

Shares the /health endpoint request mapping between security config
and MVC dispatcher. Generalizes so that instead of a marker
interface (AnonymouslyAccessibleMvcEndpoint), an MvcEndpoint
signals that it wants to control its own access rules by adding
a Principal to the @RequestMapping method parameters (more @MVC).

Fixes gh-2015 slightly differently

spring-boot-samples/spring-boot-sample-actuator/src/test/java/sample/actuator/EndpointsPropertiesSampleActuatorApplicationTests.java

@@ -69,5 +69,8 @@ public class EndpointsPropertiesSampleActuatorApplicationTests {
 		assertEquals(HttpStatus.OK, entity.getStatusCode());
 		assertTrue("Wrong body: " + entity.getBody(),
 				entity.getBody().contains("\"status\":\"UP\""));
+		System.err.println(entity.getBody());
+		assertTrue("Wrong body: " + entity.getBody(),
+				entity.getBody().contains("\"hello\":\"world\""));
 	}
 }