Update gke script to allow preempt clusters
Preemptible clusters come with a few limitations: https://cloud.google.com/compute/docs/instances/preemptible#limitations
And aren't always available for creation, but the savings is 70-80% (I believe the VM machines themselves are 80% off, and the local ssd is 70% off)
Other GCP related GCP costs remain the same. (balancers, ingress/egress)