Fix parameter parsing for unspecified target nodes#1552

Barry-Xu-2018 wants to merge 2 commits intoros2:rollingros2/rclpy:rollingfrom

Barry-Xu-2018:review/topic-fix-no-target-nodes-behaviorBarry-Xu-2018/rclpy:review/topic-fix-no-target-nodes-behaviorCopy head branch name to clipboard