Foreman » Installer

Today, we expose plugin_prefix and hammer_plugin_prefix as params in puppet-foreman, which means these get stored as answers.
However, this means that one can't restore a backup from a different OS (like from EL7 on EL8), as the prefix changes (tfm-rubygem → rubygem).

I think that users rarely (read: never) have to override our automatic detection of the prefix, so we should move that out of params (and answers).
For the rare case of a user needing to override this, we shall allow that via an "foreman::globals" class, that can be overriden via hiera.

See https://github.com/theforeman/puppet-foreman_proxy/commit/935d45ab53e59fa5424a60eb54b29ccf6b666313 for an related example in puppet-foreman_proxy