Update subctl cloud prepare for gcp to support load-balancer mode

[sridhargaddam]

Provide necessary description on how to configure the GCP cloud for load-balancer mode when using "subctl cloud prepare gcp ..."

[yboaron]

We need to decide how to label the nodes in case SM deployed with LB.
Check - submariner-io/submariner#1943

[stale]

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[dfarrell07]

The idea here is to have a dedicated flag to configure LB, instead of bad-UX just setting gateways to 0.