Install CKAN Quality Assurance plugin

This creates ansible roles for installing the CKAN Quality Assurance plugin and its dependencies: https://github.com/ckan/ckanext-qa; thanks to @sharph for the regex.

This incorporates two Ansible Galaxy roles by the indefatigable @hectcastro, which are installed via the ansible-galaxy command, and are .gitignored. Note that this means setting up the repo requires an additional step prior to `vagrant up` to install the necessary ansible roles (see the README for details). There is code for including the Ansible Galaxy stuff [here](https://github.com/azavea/nyc-trees/blob/develop/Vagrantfile#L4-L26), but I chose to leave it out for the moment so that we all can get a feel for how Ansible Galaxy works before hiding it.
Did you forget to document the virtualenv_version var?