def cleanup
      # Provisioning should have set the host 'instance' values.
      kill_instances(@hosts.map{ |h| h['instance'] }.select{ |x| !x.nil? })
      delete_key_pair_all_regions()
      nil
    end
def cleanup
      # Provisioning should have set the host 'instance' values.
      kill_instances(@hosts.map{ |h| h['instance'] }.select{ |x| !x.nil? })
      delete_key_pair_all_regions()
      nil
    end
Cleanup all earlier provisioned hosts on EC2 using the Aws::EC2 library

 It goes without saying, but a #cleanup does nothing without a #provision
 method call first.

 @return [void]