Support Spot provisioning in VM instance module

[tpdownes]

• Enable Spot provisioning model when var.spot is set
  • do not support out-dated "preemptible" model now that Spot is GA
• disable boot disk deletion by Compute Engine in favor of allowing Terraform to destroy the disk; otherwise terraform destroy is in a race condition with GCE and fails with some high %

Submission Checklist

• Have you installed and run this change against pre-commit? pre-commit install
• Are all tests passing? make tests
• If applicable, have you written additional unit tests to cover this
  change?
• Is unit test coverage still above 80%?
• Have you updated any application documentation such as READMEs and user
  guides?
• Have you followed the guidelines in our Contributing document?

[tpdownes]

Triggered validation test with Omnia that uses vm-instance module.

[tpdownes]

I am force pushing a change with this local diff relative to the branch on my fork (i.e. what you just reviewed):

diff --git a/modules/compute/vm-instance/main.tf b/modules/compute/vm-instance/main.tf
index 8cccacc..a789957 100644
--- a/modules/compute/vm-instance/main.tf
+++ b/modules/compute/vm-instance/main.tf
@@ -89,7 +89,7 @@ resource "google_compute_instance" "compute_vm" {
   boot_disk {
     source      = google_compute_disk.boot_disk[count.index].self_link
     device_name = google_compute_disk.boot_disk[count.index].name
-    auto_delete = false
+    auto_delete = true
   }

   network_interface {
diff --git a/modules/compute/vm-instance/variables.tf b/modules/compute/vm-instance/variables.tf
index d749d5e..084496e 100644
--- a/modules/compute/vm-instance/variables.tf
+++ b/modules/compute/vm-instance/variables.tf
@@ -184,4 +184,4 @@ variable "preemptible" {
   description = "Provision VMs using discounted Spot pricing, allowing for preemption"
   type        = bool
   default     = false
-}
\ No newline at end of file
+}