Support for disabled Load Balancer

[prometherion]

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/kind feature

Describe the solution you'd like

Cluster API has contracts to support externally managed Control Planes and it's widely used by several other CAPI implementations: OpenStack, AWS, Azure, and many others.

In a such scenario, the Control Plane would be backed by a third provider (Control Plane Provider, e.g.: Kamaji which is responsible for providing the Control Plane Load Balancer).

When the Load Balancer is disabled, the GCP Cluster API provider should wait for a Load Balancer address, without taking for granted the LB must be always generated by itself.

Anything else you would like to add:

This is an effort to support Hosted Control Planes also in the Google Cloud Platform, and I'm open to raising a related PR to get this done.