[SPARK-9074] [LAUNCHER] Allow arbitrary Spark args to be set.

This change allows any Spark argument to be added to the app to
be started using SparkLauncher. Known arguments are properly
validated, while unknown arguments are allowed so that the
library can launch newer Spark versions (in case SPARK_HOME points
at one).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7975 from vanzin/SPARK-9074 and squashes the following commits:

b5e451a [Marcelo Vanzin] [SPARK-9074] [launcher] Allow arbitrary Spark args to be set.