Add ckanext-disqus extension

Rename site.yml to site.yml.example for disqus config
Ansible has a ton of places where variables can be defined and it's not always clear when one should be preferred over another. I've been placing this type of variable into `hosts/hosts.vagrant`, so I'd lean toward moving this there, but if you've got a strong reason to prefer this approach, we could do it this way, as long as we're consistent.

@hectcastro do you have any insight on Ansible variable declarations and which approaches are preferable in which situations?