Don't make the mesos-*-flags.sh executable.

Review: https://reviews.apache.org/r/13205
Use 'exec' to launch mesos-slave from wrapper.

Review: https://reviews.apache.org/r/13206
Added checkpoint parameter to JobTracker.

Now it's possible to set checkpoint=true for the Hadoop framework by
specifying mapred.mesos.checkpoint=true in the Hadoop config.

Review: https://reviews.apache.org/r/13091