Security enhancements

[Abdul-Kinadiyil]

1. Ensure the public IP access is restricted through Identity aware proxy. Use private clusters as well
2. Use Preemptible VMs for the clusters so that they don't run beyond 24 hours.
3. Revert the org following org policy changes after the cluster is set up.
   constraints/compute.trustedImageProjects
   constraints/compute.vmExternalIpAccess
   constraints/compute.restrictSharedVpcSubnetworks
   constraints/compute.restrictSharedVpcHostProjects
   constraints/compute.restrictVpcPeering
   constraints/compute.vmCanIpForward