[reconfigurator] Selecting a zpool for new zones only looks at PlanningInput, not disks in the blueprint

[jgallagher]

When a new zone is added, BlueprintBuilder::sled_select_zpool() is called to choose a zpool for that zone's transient root dataset (and optionally its durable dataset). This method only looks at the sled's SledResources, which comes from the PlanningInput. It should also consider the entries in blueprint_disks to avoid emitting blueprints where a zone references a disk that is not managed by sled-agent.