[SPARK-2894] spark-shell doesn't accept flags

As @sryza reported, spark-shell doesn't accept any flags.
The root cause is wrong usage of spark-submit in spark-shell and it come to the surface by #1801
Also arguments passed to applications aren't restricted to `--*` options, so I think it makes sense to rename this to something ARGS. That would also make the variable naming here more consistent (right now it's not clear that `PYSPARK_SUBMIT_ARGS` and `APPLICATION_OPTS` are actually analogous)