[SPARK-5816] Add huge compatibility warning in DriverWrapper

The stability of the new submission gateway assumes that the arguments in `DriverWrapper` are consistent across multiple Spark versions. However, this is not at all clear from the code itself. In fact, this was broken in 20a60131, which is fortunately OK because both that commit and the original commit that added this gateway are part of the same release.

To prevent this from happening again we should at the very least add a huge warning where appropriate.

Author: Andrew Or <andrew@databricks.com>

Closes #4687 from andrewor14/driver-wrapper-warning and squashes the following commits:

7989b56 [Andrew Or] Add huge compatibility warning

core/src/main/scala/org/apache/spark/deploy/worker/DriverWrapper.scala

@@ -26,10 +26,17 @@ import org.apache.spark.util.{AkkaUtils, ChildFirstURLClassLoader, MutableURLCla
 
 /**
  * Utility object for launching driver programs such that they share fate with the Worker process.
+ * This is used in standalone cluster mode only.
  */
 object DriverWrapper {
   def main(args: Array[String]) {
     args.toList match {
+      /*
+       * IMPORTANT: Spark 1.3 provides a stable application submission gateway that is both
+       * backward and forward compatible across future Spark versions. Because this gateway
+       * uses this class to launch the driver, the ordering and semantics of the arguments
+       * here must also remain consistent across versions.
+       */
       case workerUrl :: userJar :: mainClass :: extraArgs =>
         val conf = new SparkConf()
         val (actorSystem, _) = AkkaUtils.createActorSystem("Driver",