Source domain configuration from the GitLab API

Follow-up issue on enabling different source configuration for Pages based on #210010 (comment 342953588) and #217912 (comment 344034359)

1. Implement -domain-config-source=gitlab -> serve 502 while the service is not ready
2. Remove need to call disk.New when -domain-config-source=gitlab
3. [-] Remove polling from Pages implemented in gitlab-pages!304 (merged) moved to its own issue gitlab-pages#449 (closed). Polling will help decide source for auto.
4. [-] Update documentation on -domain-config-source=gitlab usage. Done as part of #217912 (closed)