Add pydocs to mesos.py for Executor/Scheduler and driver methods.

From: Brian Wickman <wickman@gmail.com>
Review: https://reviews.apache.org/r/9572

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1459435 13f79535-47bb-0310-9956-ffa450edef68