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

I added a test called ModClusterSubsystemTestCase, which uses our subsystem testing framework which attempts to validate the description providers, this will be committed to upstream in the next few days.
To make the tests pass I had to override AdditionalInitialization.getModelValidationConfiguration() and isValidateOperations() as follows:

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


         @Override
         protected boolean isValidateOperations() {
             //TODO fix providers https://issues.jboss.org/browse/AS7-1789
             return false;
         }

Returning null basically turns off validation of the providers, and the isValidateOperations() override turns off validation of the contents of the actual operations. 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

The above is standard text as I'm going through and creating tests. The test has been @Ignored due to a further failure once validation 'passes' (i.e. the validation is not run due to the flags set)