Review moving the `GITLAB_CDN_HOST` environment variable into gitlab.yml configuration

During the API Service migration to Kubernetes we found that the GITLAB_CDN_HOST environment variable can ONLY be configured via the environment. This created a very inconsistent approach towards configuring running the software and depending on the type of configuration, could be dangerous to expose as an environment variable

Can we move this to be served via our gitlab.yml configuration procedure (gitlab.rb in Omnibus)?