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).
I'm glad, I was unsure which way to do it.  I think this is better, as then users can force an override either way.