[Web UI]Make decision order of Worker's WebUI port consistent with Master's

The decision order of Worker's WebUI port is "--webui-port", SPARK_WORKER_WEBUI_POR, 8081(default), spark.worker.ui.port. But in Master, the order is "--webui-port", spark.master.ui.port, SPARK_MASTER_WEBUI_PORT and 8080(default).

So we change the order in Worker's to keep it consistent with Master.

Author: WangTao <barneystinson@aliyun.com>

Closes #1838 from WangTaoTheTonic/reOrder and squashes the following commits:

460f4d4 [WangTao] Make decision order of Worker's WebUI consistent with Master's
[SPARK-2911] apply parent[T](j) to clarify UnionRDD code

References to dependencies(j) for actually obtaining RDD parents are less common than I originally estimated.   It does clarify UnionRDD (also will clarify some of my other PRs)

Use of firstParent[T] is ubiquitous, but not as sure that benefits from being replaced with parent(0)[T].

Author: Erik Erlandson <eerlands@redhat.com>

Closes #1858 from erikerlandson/spark-2911-pr2 and squashes the following commits:

7ffea74 [Erik Erlandson] [SPARK-2911] apply parent[T](j) to clarify UnionRDD code