Spot Instance rebalance recommendation support

[GwonsooLee]

New feature for autoscaling group came out.

Instance rebalancing occurs before 2-minutes termination notification and autoscaling will change the target spot instance with the new one available. This helps users that they no longer need to concern about the sudden termination of spot.

Further information:
https://aws.amazon.com/about-aws/whats-new/2020/11/introducing-ec2-instance-rebalance-recommendation-for-ec2-spot-instances/