Set up CKAN with Vagrant and ansible provisioning

Largely copied from a Github gist (see comment in the playbook), this
provisions a Vagrant box containing a working installation of CKAN. CKAN
is installed from a release package, rather than from source; as best I
can tell we can extend it via extensions.

I tried to use Ubuntu 14.04 initially but the package install fails, and
it turns out that the current 2.2 release of CKAN doesn't support 14.04
(https://github.com/ckan/ckan/issues/1651), so this uses Ubuntu 12.04.