[SPARK-2423] Clean up SparkSubmit for readability

It is currently non-trivial to trace through how different combinations of cluster managers (e.g. yarn) and deploy modes (e.g. cluster) are processed in SparkSubmit. Moving forward, it will be easier to extend SparkSubmit if we first re-organize the code by grouping related logic together. This is a precursor to fixing standalone-cluster mode (SPARK-2260).