Refactor Puppet bits for the nightly builds NFS

Originally created by @bertagaz on #9616 (Redmine)

Now that we introduced the tails::jenkins::artifacts_store class, we should also abstract in there the NFS part of nightly.t.b.o.

That will also be the moment where to put more logic in this class, like dealing with the File[$artifacts_store_path] questions.

Parent Task: tails/tails#9614

Edited May 15, 2020 by bertagaz