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
So here, it might be a bit nicer to do something like:

```
value match {
  case k :: "=" :: v =>
    sparkProperties(k) = v
  case _ =>
    // throw exception saying there is a bad format
}
```
Would this work?