cleanup.sh errors out if ZooKeeper not running

A line towards the end of cleanup.sh causes the script to error out if no ZooKeeper process is running on the box:

```
$ ./cleanup.sh
kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
```

...which comes from the script command:

```
kill -9 `jps | grep QuorumPeerMain| cut -f 1 -d " "`
```

Not a major issue, but it'd be a nicer experience if we made this 'kill' command dependent on the jps/grep sub-shell actually producing a pid to terminate.