Support GCP machine-image pools for burst range launches #2371

Compute Engine permits at most six instance creations from a single machine image in a rolling 60-minute window. A range template bound to one machine image therefore cannot support larger event launch bursts; retrying the same image does not increase capacity.

Provide an administrator-managed pool of equivalent, immutable GCP machine-image references for a range host binding. Validate image kind/project and explicit opt-in, select a source per range with bounded per-source launch accounting, persist the selected source for retry/idempotency, and report capacity exhaustion clearly. Keep existing single-image bindings compatible. Add synthetic tests for concurrent launches, exhausted pools, and failed/retried launches. Document the six-per-hour-per-image prerequisite and capacity sizing.

Reference: https://cloud.google.com/compute/docs/machine-images/create-instance-from-machine-image#restrictions