SPARK-2310. Support arbitrary Spark properties on the command line with ...

...spark-submit

The PR allows invocations like
  spark-submit --class org.MyClass --spark.shuffle.spill false myjar.jar

Author: Sandy Ryza <sandy@cloudera.com>

Closes #1253 from sryza/sandy-spark-2310 and squashes the following commits:

1dc9855 [Sandy Ryza] More doc and cleanup
00edfb9 [Sandy Ryza] Review comments
91b244a [Sandy Ryza] Change format to --conf PROP=VALUE
8fabe77 [Sandy Ryza] SPARK-2310. Support arbitrary Spark properties on the command line with spark-submit