Make deprecation of clustered option work.

When starting HornetQ server in standalone mode, I always get the HQ222018 log line. It took me some time to find it in the documentation and even more time to find out, that the option is actually not present in sample configuration BUT the check for the element is actually wrong in the code. This pull request fixes both. (i refrained from removing the default value of the option in the schema, but I think that would be a logical step as well).