Revert the mesos slave upstart script

This reverts the mesos-slave upstart script back to the way it was.

The su -l version looks copy+pasted from the sysv init version, and is *extremely* complicated. It doesn't work anyway because of the mismatch between SLAVE_OPTS and MESOS_SLAVE_OPTS. Does anyone use this for real?

Anyway, this is all silly to use su + start-stop-daemon + bash + exec + mesos-slave when upstart could just run mesos-slave directly. 

Would you accept a PR to make this just... run the thing?

```
${DAEMON} ${MESOS_SLAVE_OPTS}
```

I mean, why are we making it more complicated than it needs to be? 

cc @mrtyler