Support Multi-Zone Managed Instance Group in GCP

Problem to solve

Currently the plugin cannot use Multiple zones Managed Instance Group (MIG) in GCP. When I try to use MIG with Multiple zones enabled, I get an error:

failed to update executor: initializing taskscaler: creating taskscaler: initializing provisioner: reconciling with instance group: rpc error: code = Unknown desc = googleapi: Error 404: The resource 'projects/test-gitlab-runners/zones/europe-north1/instanceGroupManagers/test-gitlab-runners-instance-group' was not found, notFound executor=docker-autoscaler max_builds=1 runner=t2_xxx

When I try to use the Single zone MIG, it works fine.

Implementation proposal

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