DR: update hostname and DNS records

If a disaster occurs and a secondary is promoted, it needs to replace the existing primary at it's previous domain. This will require DNS changes outside of GitLab, and the secondary external URL needs to be updated.

Proposal

• update the external URL when running promote-secondary-to-primary
• allow continued access through the secondary external URL else it will break users who have updated their git URLs to use the secondary
• provide feedback to the operator to update the DNS settings to point to the new secondary