Migrate Registry to GCP immediately

As discussed with @jarv

As part of the operational readiness review, taking a small, relatively simple service and running it on GCP would be a great way of proving that our new production environment is working well.

registry.gitlab.com is a perfect candidate.

• Leave object storage on AWS S3 as it is now
• Spin up registry fleet on GCP
• Switch DNS over to new registry
• Ensure that everything is working correctly. If it's not, switching back to the old Azure instance is as easy as making a DNS change, so there's little risk involved.

Requires #175 (closed) #393 (closed)