Add support for nested virtualization on GCE instances

[jwmay2012]

What type of PR is this?
/kind feature

What this PR does / why we need it:
Adds support for enabling nested virtualization on GCE instances via the enableNestedVirtualization field in GCPMachineSpec. This maps directly to GCP's AdvancedMachineFeatures.EnableNestedVirtualization API field.

This enables running VMs inside GCE instances for container sandboxes, KVM, QEMU, etc.

Special notes for your reviewer:
This is a simple boolean field that maps directly to the GCP API. The field must be set at instance creation time and cannot be changed after creation (GCP limitation).

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Add support for nested virtualization on GCE instances. `GCPMachine.Spec.EnableNestedVirtualization` allows running VMs inside GCE instances.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jwmay2012
Once this PR has been reviewed and has the lgtm label, please assign fabriziopandini for approval. For more information see the Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[k8s-ci-robot]

Hi @jwmay2012. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[netlify]

✅ Deploy Preview for kubernetes-sigs-cluster-api-gcp ready!

Name	Link
🔨 Latest commit	06707a4
🔍 Latest deploy log	https://app.netlify.com/projects/kubernetes-sigs-cluster-api-gcp/deploys/68ee7e9b6ea130000815f43c
😎 Deploy Preview	https://deploy-preview-1537--kubernetes-sigs-cluster-api-gcp.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[justinsb]

/ok-to-test

[jwmay2012]

/retest

[jwmay2012]

Feedback addressed. Let me know if there's anything else :)