(CDAP-588) Temp fix for Spark ClassLoading issue in standalone mode

- Set the context classloader for the SparkSubmit to CDAP system ClassLoader
  - This make all system classes available for Spark
  - User classes is loaded from a flatten uber jar created on the fly
Merge pull request #606 from caskdata/feature/fix-spark-classloader

(CDAP-588) Fix Spark ClassLoader issue.