SPARK-1352: Improve robustness of spark-submit script

1. Better error messages when required arguments are missing.
2. Support for unit testing cases where presented arguments are invalid.
3. Bug fix: Only use environment varaibles when they are set (otherwise will cause NPE).
4. A verbose mode to aid debugging.
5. Visibility of several variables is set to private.
6. Deprecation warning for existing scripts.