Previously it was possible to bind a bean to the root prefix by just
adding `@EnableConfigurationProperties` with the class of said bean.

 This use case is misleading and prevents any meta-data to be generated
 for that object since the annotation processor reacts on the presence of
 the `@ConfigurationProperties` annotation.

 If a class is included in the list of configuration properties bean to
 create via the `@EnableConfigurationProperties` annotation we now make
 sure that the `@configurationProperties` annotation is present on it.

 Closes gh-3460