use path_join filter to avoid //

(cherry picked from commit a0a9ff70)

Fails on sasha:

TASK [plone.maintenance : Update Plone project.] ******************************* 14:54:54 fatal: [starzel04]: FAILED! => {"msg": "An unhandled exception occurred while templating '{{ (plone_base_path, plone_instance_name) | path_join }}'. Error was a <class 'ansible.errors.AnsibleError'>, original message: template error while templating string: no filter named 'path_join'. String: {{ (plone_base_path, plone_instance_name) | path_join }}"} 14:54:54 to retry, use: --limit @/var/lib/rundeck/starzel.ansible/retry/dynatest.starzel.de.retry

Merge as soon as sasha server is new.