Added firewall feature.

Hello Tom,

Thanks for your puppet-module. Since we use in our organization a firewall-module, I've added a firewall-file, which opens the port of the NFS-service if its running.
I've also added a file.exists-check to make sure that the firewall-module is used or else just to ignore the firewall-rules.

I'm planning to request a pull for the puppetlabs-firewall-module to add a boolean firewall.exist-facter which says whether a firewall exists or not. So the file.exists-check can be replaced by a check which is delivered by the firewall-module.

Hopefully these changes can be pulled in your repository.

Greatings,

Abdelouahed Haitoute