Ensure GCP control plane machines are assigned to instance group

User Story

As an OpenShift admin on GCP I want to replace my control plane node. Currently, the new control plane machine does not get assigned instance group. This is prevents the internal load balancer working on the new node until you set it manually using the GCP console. This prevents automatic control plane replacement using the CPMSO on GCP.

Background

CAPI implementation:
_https://github.com/openshift/cluster-api-provider-gcp/blob/f6b71187180cb35b93caee08eddbccb66cb28ab6/cloud/services/compute/instances/reconcile.go#L141_

Steps

• Create abstraction around GCP instance group API
• Register machine to instance group on create
• Remove machine from instance group on delete

Stakeholders

• Cluster Infrastructure (CPMSO)

Definition of Done

• Control plane machines are reconciled into correct instance group

• Docs

• N/A

• Testing

• Manual node replacement as CPMS is currently disabled on GCP