Use puppet-git's tails Git repo in reprepro config generation script

Reprepro scripts that need to fetch the main Tails git repo must use the one on puppet-git so that network troubles are handled at the git level and we don’t get annoyed by tons of emails when cronjobs fail.

This means passing another $git_remote parameter to the tails::reprepro::custom class, managing a SSH key pair with Puppet, and configuring Gitolite to give read-only access to that key.

Parent Task: #11869

Original created by @bertagaz on 8509 (Redmine)