[FR] Implement disk deletion backoff

[Temikus]

In run_instance logic, disk cleanup should implement exponential backoff. Probably by inserting retriable somwhere over here:

        def cleanup_disk(disk_name, env)
          zone = env[:machine].provider_config.zone
          autodelete_disk = env[:machine].provider_config.get_zone_config(zone).autodelete_disk
          if autodelete_disk
            disk = env[:google_compute].disks.get(disk_name, zone)
            disk.destroy(false) if disk
          end
        end

• this is a good opportunity to add some debug logging around the issue. #148 on why this is needed