java_gen: use correct type+default for of_flow_mod.out_group

Reviewer: @rlane @vaterlaus

This pull request corrects the API type and default value for the "outGroup" property of the OFFlowMod classes. It is now an instance of OFGroup (as opposed to a long), and defaults to OFGroup.ANY. The value of the field is relevant only for of_flow_delete*, in which case "ANY" is the "don't care" setting. 

I am not bumping the version, because the out_group field isn't yet set anywhere in our existing code outside of the unit tests, so the API change does not break anything.