Add option to create an InstanceGroup to be used for bootstrap instances

[bfournie]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Add a new configuration field to specify that an InstanceGroup should be created for a bootstrap instance. When configured, the InstanceGroup will be created in the first zone and if a bootstrap Instance is created it will be added to this InstanceGroup.

Which issue(s) this PR fixes *(optional, in `fixes #(, fixes #
Fixes #1265

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Added a new field `CreateBootstrapInstanceGroup` to the `LoadBalancer` struct which configures the Internal LoadBalancer. By default, this field will not be set. When set to 'true' an InstanceGroup will be created named <cluster-id>-bootstrap. If a bootstrap Instance is created it will be added to this InstanceGroup.

[k8s-ci-robot]

Hi @bfournie. 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.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

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[damdo]

/ok-to-test

[richardcase]

API diff changes are in the non-API packages so:

/override pull-cluster-api-provider-gcp-apidiff

[k8s-ci-robot]

@richardcase: Overrode contexts on behalf of richardcase: pull-cluster-api-provider-gcp-apidiff

In response to this:

API diff changes are in the non-API packages so:

/override pull-cluster-api-provider-gcp-apidiff

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.