def modify_instance_groups(instance_group_config)
      params = {
        :operation => 'ModifyInstanceGroups',
        :instance_groups => instance_group_config.map { |k, v| {:instance_group_id => k, :instance_count => v} }
      }
      aws_result = @aws_request.submit(params)
      yield aws_result if block_given?
    end
def modify_instance_groups(instance_group_config)
      params = {
        :operation => 'ModifyInstanceGroups',
        :instance_groups => instance_group_config.map { |k, v| {:instance_group_id => k, :instance_count => v} }
      }
      aws_result = @aws_request.submit(params)
      yield aws_result if block_given?
    end
Set the number of instances in the specified instance groups to the
 specified counts.  Note that this modifies the *request* count, which
 is not the same as the *running* count.  I.e. you request instances
 and then wait for them to be created.

 Takes a {} of instance group IDs => desired instance count.

   {"ig-1" => 40, "ig-2" => 5, ...}