Resolve puppet warnings

The following warning were emitted while compiling the catalog

```
Warning: Variable access via 'slave_user' is deprecated. Use '@slave_user' instead. template[/home/USERNAME/Code/work/puppet/modules/jenkins/templates/jenkins-slave.erb]:18
   (at /home/USERNAME/Code/work/puppet/modules/jenkins/templates/jenkins-slave.erb:18:in `result')
Warning: Variable access via 'slave_user' is deprecated. Use '@slave_user' instead. template[/home/USERNAME/Code/work/puppet/modules/jenkins/templates/jenkins-slave.erb]:19
   (at /home/USERNAME/Code/work/puppet/modules/jenkins/templates/jenkins-slave.erb:19:in `result')
```

To reduce the noise generated (and be up-to-date with current puppet standards), I changed the variables to their instance-variable equivalent.