Refactor the Actuator MVC configuration to allow more customization

There is a new spring.factories entry for
org.springframework.boot.actuate.autoconfigure.EndpointWebMvcConfiguration
which loads extra beans into the MVC config for the Actuator.
If the management context is a child context all the beans go in the
child (except the Spring Security filter still). A big bonus is that
you can add WebConfigurerAdapters to configure static resources etc.
A new component called ManagementContextResolver can be used to
locate the ApplicationContext for the MVC endpoints.

Fixes gh-3345

spring-boot-samples/spring-boot-sample-actuator/src/main/resources/application.properties

@@ -1,5 +1,5 @@
 # logging.file: /tmp/logs/app.log
-# logging.level.org.springframework.security: INFO
+# logging.level.org.springframework.security: DEBUG
 management.address: 127.0.0.1
 #management.port: 8181
 endpoints.shutdown.enabled: true