[SPARK-1774] Respect SparkSubmit --jars on YARN (client)

SparkSubmit ignores `--jars` for YARN client. This is a bug.

This PR also automatically adds the application jar to `spark.jar`. Previously, when running as yarn-client, you must specify the jar additionally through `--files` (because `--jars` didn't work). Now you don't have to explicitly specify it through either.

Tested on a YARN cluster.