Fix messaging description providers for the improved validation in jboss-as-subsystem-test

I am adding some more validation of the model. This will be committed to upstream in the next few days. To make the messaging tests pass I had to override AdditionalInitialization.getModelValidationConfiguration() as follows:

@Override
           protected ValidationConfiguration getModelValidationConfiguration() {
               return null;
           }

Returning null basically turns off validation of the providers. What I have added is a minimum of what is required for subsystem testing, the test framework allows you to do a lot of other stuff, see the jmx subsystem test for some examples.

The wip lives at https://github.com/kabir/jboss-as/tree/subsystem-test-model-fixes