refs #7063 - change cache path so installer (root) doesn't conflict with master (puppet) user

On Fedora 19 at least, our tests are failing as the installer is causing /var/lib/puppet/server_data to be owned by root (user under which the installer runs), which means the Puppet master (which is "puppet") can't manage it.

    Sep  2 03:51:09 server-163 puppet-master[10771]: failed to set mode 755 on /var/lib/puppet/server_data: Operation not permitted - /var/lib/puppet/server_data

My putting the cache under this directory was a bad idea, so moved back to the shared vardir.