[#55632912] using rssh to restrict user to rsync

Added puppet-forge module rssh. We evaluated other options like ssh
authorize keys and rrsync. But rssh seems to be the better one.
Overiding user shell to rssh shell and allowing only rsync acess
Merge pull request #10 from alphagov/55632912_restrict_mirror_user_to_rsync_only

[#55632912] using rssh to restrict user to rsync