Follow-up from "Make hostname configurable for smartcard authentication"

The following discussion from !26411 (merged) should be addressed:

• @twk3 started a discussion:

  Settings.gitlab['host'] is being used by the smartcard settings, before it has been defaulted in 1_settings.rb. That means that if it was unset or set using the environment variable, we will get an error:

  │ migrations NoMethodError: undefined method `[]' for nil:NilClass
  │ migrations /srv/gitlab/config/initializers/1_settings.rb:80:in `block in <main>'
  │ migrations /srv/gitlab/lib/gitlab.rb:95:in `ee'