Rename VanillaHealthIndicator to Application...

Rename `VanillaHealthIndicator` to `ApplicationHealthIndicator` and
changed the exposed bean name from `statusHealthIndicator` to
`applicationHealthIndicator`.

This provides less confusing JSON output:

  {"status":"UP","application":{"status":"UP"}}

vs:

  {"status":"UP","status":{"status":"UP"}}

Fixes gh-1291

spring-boot-samples/spring-boot-sample-actuator-ui/src/test/java/sample/actuator/ui/SampleActuatorUiApplicationPortTests.java

@@ -75,7 +75,8 @@ public class SampleActuatorUiApplicationPortTests {
 		ResponseEntity<String> entity = new TestRestTemplate().getForEntity(
 				"http://localhost:" + this.managementPort + "/health", String.class);
 		assertEquals(HttpStatus.OK, entity.getStatusCode());
-		assertEquals("{\"status\":\"UP\"}", entity.getBody());
+		assertEquals("{\"status\":\"UP\",\"application\":{\"status\":\"UP\"}}",
+				entity.getBody());
 	}
 
 }