[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