refs #7234 - install foreman-release-scl on EL clones

Depends on https://github.com/theforeman/foreman-packaging/pull/364

Foreman-specific SCL release package will replace per-distro SCL configuration
as it's more complex and more up to date.

configure_*_repo logic moved into parameter defaults which also allows users to
override the logic (e.g. on RHEL self-support, it can now be enabled).
Exactly. Ideally there is no distro specific logic inside classes other than params.pp.