Vagrantfile/pergamon: Add puppet_vardir fact to reasonable value
Or the default value targets an inexistant /var/cache/puppet folder:
==> pergamon: Error: Evaluation Error: Error while evaluating a Function Call, Could not find any files from /var/cache/puppet/ssl/certs/pergamon.softwareheritage.org.pem (file: /tmp/vagrant-puppet/environments/production/site-modules/profile/manifests/puppet/master.pp, line: 29, column: 30) on node pergamon.softwareheritage.org
or
==> pergamon: Error: Removing mount "le_certs": /var/cache/puppet/letsencrypt_exports does not exist or is not a directory
Related to T3325