Fixes #8210 - Adding caching for smart-proxy puppet classes using Stefan Julin's implementation of cache

It shouldn't be 'foreman' at all, this is the smart proxy.

I think it'd be best to have it default (or even be hardcoded) to the installation dir + '/cache/', then in the packaging we can symlink /usr/share/foreman-proxy/cache to /var/cache/foreman-proxy in a similar way to how we do other directories (inc in Foreman core).