Avoid the use of SETTINGS[:fqdn]
SETTINGS[:fqdn] is used in two places:
https://github.com/Katello/katello/blob/d9f5d173cb8a604caae3ad20c3f237d06c0f846b/app/controllers/katello/api/rhsm/candlepin_proxies_controller.rb#L269 (and its test case)
The downside of this is the application is assuming the system hostname is also the service name. When running behind a load balancer a common pattern is app1.example.com and app2.example.com both serving app.example.com. Because of that
Setting[:foreman_url] exists and should be relied on for the canonical Foreman URL.