Critical Description
When Spark executors are deployed on Mesos, the Mesos scheduler no longer respects the "spark.executor.extraClassPath" configuration parameter.
MesosCoarseGrainedSchedulerBackend used to use the environment variable "SPARK_CLASSPATH" to add the value of "spark.executor.extraClassPath" to the executor classpath. However, "SPARK_CLASSPATH" was deprecated, and was removed in this commit https://github.com/apache/spark/commit/8f0490e22b4c7f1fdf381c70c5894d46b7f7e6fb#diff-387c5d0c916278495fc28420571adf9eL178.
This effectively broke the ability for users to specify "spark.executor.extraClassPath" for Spark executors deployed on Mesos.
