Make cloudflare DNS terraformable

Since we are keeping seperate zones in cloudflare for gitlab.com and staging.gitlab.com for cloudflare configuration purposes we are going to keep the split as it is right now. This means we can extend the dns environment in terraform to allow managing the staging.gitlab.com. zone via the cloudflare provider without impacting production.

This also resolves diversion from production, because even though the underlying DNS is different, there is no difference in managing it, then.

We should be able to keep the same structure for configuring that via the json files as we do for route53.