Adding GCPMachinePool Functionality

[BrennenMM7]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This PR Implements GCPMachinePool using Managed Instance Groups. This is needed to support the CAPI MachinePools object. Feature will allow cluster operators to deploy GCP MIG's to their CAPG clusters to take advantage of both Google Autoscaling of Compute Instances or utilizing cluster autoscaler to dynamically provision node resources.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #297,#1005

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Feature: adds MachinePool/GCPMachinePool Objects to deploy Managed Instance Group services. 

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@BrennenMM7: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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[cpanato]

/ok-to-test
/assign @richardcase

thanks for your PR, an initial comments

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[damdo]

I've had a chat during the CAPG meeting with @BrennenMM7.

@BrennenMM7 and team would like some thoughts on the overall design of the API, mainly from maintainers.
Once that is agreed on they can write E2Es to prove this works and ensure regression checks on future changes.

/cc. @cpanato @richardcase @dims

[k8s-ci-robot]

PR needs rebase.

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[damdo]

@BrennenMM7 this will need a rebase :)

[salasberryfin]

Would we be able to avoid having to watch KubeadmConfig? This would prevent anyone enabling MachinePool from selecting the bootstrap provider of their choice and force them to use Kubeadm. This is something we've realized happens in CAPZ and there's an open issue to address this. Other providers, like CAPA, don't have this dependency on Kubeadm.

[richardcase]

A great point @salasberryfin 👍

[damdo]

+1, thanks for catching this!

[cpanato]

+1

[BrennenMM7]

This is a great point, I'd love to abstract the mechanism away from the kubeadm bootstrap provider lock-in. Any idea how we'd enqueue a reconcile from a token rotation when two different bootstrap providers are being utilized 🤔

[cpanato]

need rebase

[BrennenMM7]

@damdo @cpanato PR has been rebased thanks 💯

[k8s-ci-robot]

@BrennenMM7: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-gcp-verify	3f4e888	link	true	/test pull-cluster-api-provider-gcp-verify
pull-cluster-api-provider-gcp-test	3f4e888	link	true	/test pull-cluster-api-provider-gcp-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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