SPARK-4447. Remove layers of abstraction in YARN code no longer needed after dropping yarn-alpha

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3652 from sryza/sandy-spark-4447 and squashes the following commits:

2791158 [Sandy Ryza] Review feedback
c23507b [Sandy Ryza] Strip margin from client arguments help string
18be7ba [Sandy Ryza] SPARK-4447