Takes a master instance type (string), a slave instance type
        (string), and a number of instances. Returns a comparable dict
        for use in making a RunJobFlow request.

def _build_instance_count_and_type_args(self, master_instance_type,
                                            slave_instance_type, num_instances):
        """
        Takes a master instance type (string), a slave instance type
        (string), and a number of instances. Returns a comparable dict
        for use in making a RunJobFlow request.
        """
        params = {
            'Instances.MasterInstanceType' : master_instance_type,
            'Instances.SlaveInstanceType' : slave_instance_type,
            'Instances.InstanceCount' : num_instances,
            }
        return params