Improve handling of custom Ruby installation

[matze]

The custom installation of Ruby via rvm, while pretty straight-forward, requires a range of improvements in order to increase it's reliability - e.g.:

• Distribute GPG keys as Puppet resource within the infrastructure repository
• Distribute any scripts as Puppet resources as well, instead of e.g. downloading them prior to execution
• Ensure the Ruby installation will not conflict with the system's one, in order to not break Puppet for the next run
• Ideally make the entire Ruby-setup re-usable (read: introduce a separate ruby module), in order to allow for easier integration of Ruby software in the future
• Ensure necessary parameters can be set via Hiera (e.g. the Ruby version)

In fact the entire discourse module seems to be broken right now. Yet we have known about some shortcomings before and agreed on improving when necessary already, including a more sophisticated Ruby setup. See #2471 and the associated ​code-review.