Create an internal load balancer for registry

Registry doesn't have an internal API endpoint, but we make internal API requests from Sidekiq. Currently this depends on us having Registry running in the same cluster as Sidekiq, see delivery#1378 (comment 457763266).

One solution to this would be to create an internal LB, like we do for front-end, but instead attach the registry HAProxy servers. Once we have this internal load balancer, we can then use it for accessing the registry internal API from sidekiq.

The downside to this approach is that this will mean we will need to keep HAProxy to route requests (from different AZs) from sidekiq (which is regional) to registry (which is zonal).

It might be better for us to run registry in the regional cluster for the purpose of handling internal API traffic.