Blueprint planner: Upstream network configuration should come from PlanningInput, not the parent blueprint #9040
Description
When determining the configuration for new Nexus and boundary NTP zones, the planner currently scans the parent blueprint for existing Nexus / boundary NTP zones to copy their upstream networking details (DNS servers and NTP servers). This is a byproduct of our inability to change them after rack setup time. We already have an issue for allowing operators to update them (#3272). For reconfigurator, we'll also need to change this logic to read these details from PlanningInput instead of scanning the parent blueprint.
As a half step, we could consider moving the "scan the parent blueprint for upstream networking config" bits out of the planner and into the construction of PlanningInput. This would simplify the planner a bit (and maybe remove some error cases we had to work around in some tests?) and put things in a better place for when we do the #3272 work (at which point we'd only change the construction of PlanningInput, instead of having to do that and update the planner concurrently).