Support bluegreen deployment of multi ELB on autoscaling group

If autoscaling group have several different ELB, you should specify array property of chain_elb. ecs-formation can swap chain_elb ELB group with main ELB group at the same time.

(path-to-path/test-ecs-formation/bluegreen) $ vim test-bluegreen.yml
blue:
  cluster: test-blue
  service: test-service
  autoscaling_group: test-blue-asg
green:
  cluster: test-green
  service: test-service
  autoscaling_group: test-green-asg
primary_elb: test-elb-primary
standby_elb: test-elb-standby
chain_elb:
  - primary_elb: test-internal-elb-primary
    standby_elb: test-internal-elb-standby