Document DNS assumptions/setup for Istio gateways

[mjnagel]

To properly deploy and access core services in a production environment DNS must be setup to point to Istio provisioned loadbalancer services. This document notes the requirement of a load balancer provisioner, but we do not currently document the DNS assumptions for routing to virtual services provisioned by the operator.

Definition of done: Documentation should be added to the deployment section that covers the DNS assumptions of core. In short DNS must be setup to route *.<domain> and *.admin.<domain> to the respective load balancers. Examples could be provided for something like AWS ELBs, but in general this documentation should be more general. This doc could also note the handling of DNS for the demo bundle/uds.dev domain.