executor can use a different sparkHome from Worker 

We'd really like to be able to specify a different sparkHome for an exector, without having to restart the Workers on the cluster.  This would really make it much easier for us to test out small changes for spark -- we'd like to simply leave the Master/Workers running on the cluster, and be able to try out different versions of spark for the executors by setting sparkHome.  We really don't want to just replace the contents of the sparkHome dir, b/c we'd like to have multiple versions around at the same time -- there may be other things using the cluster at the same time as I test out spark versions.

I realize this isn't foolproof -- eventually there may be incompatible versions of the Worker & Executor.  But I'd still like to be able to delay restarting the cluster as much as I can.

I am not sure how this relates to the mesos cluster modes ...

(Without this change, isn't the sparkHome arg to SparkContext completely ignored in standalone clusters?)