Terminate executors that aren't needed.

If we launch an executor and then kill the task immediately after, make
sure we also terminate the executor when there are no other tasks.
Make sure you clean up the cgroups test.

Sometimes the cgroup test fails because the directory is already there.
This should ensure that it gets cleanup up in that case before we exit.
Turn down noise from ZooKeeper.
Terminate correct tasks when a slave disconnects.

Previously, when a slave disconnected all tasks for that framework would
be removed and it would result in a bad state for a given framework.  In
the case of Hadoop, it would result in a bunch of zombie tasks running
on the slaves which never terminate.