Add support for Spot VMs

[jwmay2012]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Adds support for GCP Spot VMs, which are distinct from preemptible vms. Spot VMs are newer.
https://cloud.google.com/compute/docs/instances/spot
They no longer have a mandatory 24 hr limit.

Special notes for your reviewer:

Modeled PR after the addition of preemptible vms.

Documentation that specifies details on provisioningModel, SPOT and STANDARD--
https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_instance#provisioning_model
https://cloud.google.com/compute/docs/instances/create-use-spot#create

Relevant docs about field logic--

"provisioningModel": "SPOT" in the Compute Engine API

---------------

If the output includes the provisioningModel field set to SPOT, the VM is a Spot VM.

Otherwise, if the output includes the provisioningModel field set to standard or if the output omits the provisioningModel field:
- If the output includes the preemptible field set to true, the VM is a preemptible VM.
- Otherwise, the VM is a standard VM.

TODOs:

• squashed commits
• includes documentation
• adds unit tests
  No tests exist for preemptible vms. This change seems pretty simple and straight forward and doesn't cause any changes unless the new field is used.

Release note:

Add support for Spot VMs. `GCPMachineTemplate.Spec.ProvisioningModel = "Spot"`

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[k8s-ci-robot]

Hi @jwmay2012. Thanks for your PR.

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[salasberryfin]

/ok-to-test

Thanks for working on this @jwmay2012!

[salasberryfin]

Should we also explicitly set the ProvisioningModel to Standard when not null or is it implicitly set by GCP?

[jwmay2012]

I do believe GCP defaults to STANDARD. All documentation seems to be focused around specifying SPOT and otherwise ignoring the field.

I also figured it was lower risk to maintain the existing behavior when the field is not set, allowing the GCP default to continue to be used.

[salasberryfin]

The CAPG book contains a section on using Preemptible VMs (book source files can be found here). Perhaps it makes sense to also document Spot VMs and what happens with each combination of Preemptible + ProvisioningModel, as you specified on type definition. What do you think?

[jwmay2012]

The CAPG book contains a section on using Preemptible VMs (book source files can be found here). Perhaps it makes sense to also document Spot VMs and what happens with each combination of Preemptible + ProvisioningModel, as you specified on type definition. What do you think?

Makes sense. I'll add the specifics of Preemptible + ProvisioningModel to the docs I've already made. I believe Preemptible gets ignored and ProvisioiningModel overrides. This is implicit in the documentation, but I'll confirm first.