Align expectations of various JMX configurations

They all want to create an MBeanServer and when that happens
user sees no MBeans, or sometimes just one set (Spring Core,
Spring Integration or Spring Boot). To harmonise them we
create a @Bean of type MBeanServer and link to it in the
other autoconfigs

Fixes gh-1046

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

@@ -1,2 +1,3 @@
 logging.file: /tmp/logs/app.log
-service.greeting: Hello
+service.greeting: Hello
+debug: true