Load configuration from YAML

Rather than storing all the configuration in actual committed files, we should store it in a separate YAML file then load this in both the Vagrantfile and the Puppet manifests. This YAML file should live in the content directory (as it's per-project).

* Puppet: stdlib includes the `loadyaml` function: `$myhash = loadyaml('/etc/puppet/data/myhash.yaml')`
* Vagrant: We can use the Ruby YAML library, Mozilla's [Kuma project](https://github.com/mozilla/kuma/blob/master/Vagrantfile) has a good example to base off