Factorize the current logic to determine whether use 'environments' as
D-Bus registration option (rather than 'environment') in an own
function, so it is easier to read it and maintain it.
With the small helper function in place, extend the logic to support
CentOS: it is in practice the same as the RHEL one, with an additional
check to support CentOS Stream 8 (which is a rolling release, and not
versioned).
