Use a Puppetfile instead of submodules

A best practice when using is chef or puppet is to use a Cheffile/Puppetfile where you define all of your dependencies, instead of adding submodules.
I've included a Puppetfile, removed the submodules and updated the readme so users know how to download the correct Puppet modules through the _librarian-puppet_ gem.

Another idea would be to move all modules from `puppet/modules` to `module` and the manifests from `puppet/manifests` to `manifests`. What do you guys think about that?