Add additional variables to ensure scratch disks for templates

[symbiont-jon-bogaty]

Instance templates do not in their current form allow creation of scratch disks. The missing parameters mean that you end up defaulting to incompatible flags when you try to setup NVME, etc... This is an attempt to add in the missing parameters. With it I can setup an instance template with a scratch disk.

[morgante]

Unfortunately adding these is a breaking change because Terraform currently requires that all object values be filled in.

A few more places will need to be updated:

• https://github.com/terraform-google-modules/terraform-google-vm/blob/master/modules/preemptible_and_regular_instance_templates/variables.tf#L83

• terraform-google-vm/examples/instance_template/additional_disks/main.tf

  Line 31 in 48a9b07

  additional_disks = [

• terraform-google-vm/examples/mig/full/variables.tf

  Line 124 in 5926dfc

  variable "additional_disks" {