Allow for servers in the default zone to be included by default

Previously either we had zone grouped servers, or non-grouped servers using the key default. This allows for a combination of both zone grouped servers and servers in the default zone, so that they will both be included.

This is needed for the K8s migration gitlab-com/gl-infra/delivery#1060 (closed) of the first front-end service, where we will be adding a GKE internal load balancer for selected backends (which don't have a zone preference).