Fix zk:// url creation in /etc/mesos/zk

The template file is generating a trailing ',' before the path in the /etc/mesos/zk file like this:

```
zk://zk1.aster.is:2181,zk2.aster.is:2181,zk3.aster.is:2181,/mesos
```

The fix just removes the trailing comma to make the output:

```
zk://zk1.aster.is:2181,zk2.aster.is:2181,zk3.aster.is:2181/mesos
```