Reset the thread classLoader in the Mesos Executor

This is because Mesos calls it with a different environment or something, the result is that the Spark jar is missing and it can't load classes.

This fixes http://apache-spark-user-list.1001560.n3.nabble.com/java-lang-ClassNotFoundException-spark-on-mesos-td3510.html

I have no idea whether this is the right fix, I can only confirm that it fixes the issue for us.