Mount or install Azavea's fork of ckanext-datajson

Apparently it's Ansible's way of discouraging placement of variables in inventory files: https://groups.google.com/forum/#!topic/ansible-project/b-LtXWtsPnE

I knew placing them in group_vars was preferred but figured that for a small project like this it would be cleaner to keep everything in one spot. Guess not; I'll migrate them to `group_vars` so we don't have to jump through any more hoops like this.