Parametize CKAN's repository.

This was a problem faced by willhelmbot at
http://stackoverflow.com/questions/16242798/how-to-link-a-ckan-installation-in-vagrant-to-a-cloned-repo-in-the-host-os/16243862#16243862

He wanted to clone his own repository, instead of the official CKAN's, and also
wanted to clone it inside /vagrant/ckan, so he could do changes using the host
OS.

This solves both.