title: property in subclass of DatabaseConfiguration can not be found in Dropwizard yaml (hierarchical issue)
body: <p>I want to know. Can i do this or a derivation of it? In my Dropwizard config.yml I wanted to extend databaseConfiguration class so to add another property to the config.yml as below</p>
  
  <p>config.yml</p>
  
  <pre><code>database
    driverClass: org.postgresql.Driver
    user: user1
    password: password
    exampleProperty: property1 (Tried a few other permutations)
  </code></pre>
  
  <p>Assume theres a DatabaseConfiguration class that has fields with getters and annotations for driverClass, user, password etc</p>
  
  <pre><code>public class AppConfiguration extends Configuration {
    @Valid
    @NotNull
    @JsonProperty
    private DatabaseConfiguration database = new DatabaseConfiguration();
  
    public DatabaseConfiguration getDatabaseConfiguration() {
      return database;
    }
  
    @Valid
    @NotNull
    @JsonProperty
    private ExtendedDatabaseConfiguration extDatabase;
    public ExtendedDatabaseConfiguration getExtendedDatabaseConfiguration() {
       return extDatabase;
    }
  
    public class ExtendedDatabaseConfiguration extends DatabaseConfiguration {
    @Valid
    @NotNull
    @JsonProperty
    private String exampleProperty;
    public String getExampleProperty() { return exampleProperty; };
    }
  </code></pre>
  
  <p>The main class Server.java passes the ExtendedDatabaseConfiguration to the Hibernate Bundle via the call (formerly passed the DatabaseConfiguration)</p>
  
  <pre><code>    private final CustomHibernateBundle&lt;AppConfiguration&gt; hibernateBundle = new CustomHibernateBundle&lt;AppConfiguration&gt;() {
      @Override
      //Giving the subclass holding the new database property to the CustomHibernateBundle
      public DatabaseConfiguration getDatabaseConfiguration(AppConfiguration configuration) {
          return configuration.getExtendedDatabaseConfiguration();
      }
  };
      SimpleModule simpleModule = new SimpleModule();
      simpleModule.addAbstractTypeMapping(DatabaseConfiguration.class, ExtendedDatabaseConfiguration.class);
      bootstrap.getObjectMapperFactory().registerModule(simpleModule);
  </code></pre>
  
  <p>This is all in order for CustomHibernateBundle to then use this field as it pleases.</p>
  
  <p>Whatever i tried either i got the error jackson.databind.exc.UnrecognizedPropertyException or i get the following</p>
  
  <pre><code>Exception in thread "main" com.yammer.dropwizard.config.ConfigurationException: powaaim.yml has the     following errors:
  * extDatabase may not be null (was null)
  * exampleProperty may not be null (was null)
  </code></pre>
  
  <p>Just to note. I have no access to modify DatabaseConfiguration but i can extend it. Wondering whether this can be achieved so that i can define more database configuration properties via this methodology.</p>
  
  <p>Or is there just a simpler way?</p>