Delete VM when preempted

Adds `--instance-termination-action=DELETE` to delete instance when
preempted. Without this, when preemptible VM is interrupted it
leaves the instance in shutdown state with disk still incurring
cost.

To use that flag I also had to switch to spot VM provisioning, which
costs the same, have the same base behavior and a few more features.

Spot VMs unlike preemptible ones do not have a maximum runtime
length, so I've picked 3d max runtime as that's already used in the
script as "max workflow runtime".

Author: p2004a

action.sh

@@ -192,7 +192,7 @@ function start_vm {
   image_family_flag=$([[ -z "${image_family}" ]] || echo "--image-family=${image_family}")
   disk_size_flag=$([[ -z "${disk_size}" ]] || echo "--boot-disk-size=${disk_size}")
   boot_disk_type_flag=$([[ -z "${boot_disk_type}" ]] || echo "--boot-disk-type=${boot_disk_type}")
-  preemptible_flag=$([[ "${preemptible}" == "true" ]] && echo "--preemptible" || echo "")
+  preemptible_flag=$([[ "${preemptible}" == "true" ]] && echo "--provisioning-model=SPOT --instance-termination-action=DELETE --max-run-duration=3d" || echo "")
   ephemeral_flag=$([[ "${ephemeral}" == "true" ]] && echo "--ephemeral" || echo "")
   no_external_address_flag=$([[ "${no_external_address}" == "true" ]] && echo "--no-address" || echo "")
   network_flag=$([[ ! -z "${network}"  ]] && echo "--network=${network}" || echo "")