ecs/host_group: automate instance updates

[mskrajnowski]

Currently only the launch configuration is updated and the admin has to manually update the instances by draining ECS tasks and replacing EC2 instances. Would be nice to automate this, so we can make AMI or other configuration updates easily and without downtime.

https://aws.amazon.com/blogs/compute/how-to-automate-container-instance-draining-in-amazon-ecs/
https://aws.amazon.com/blogs/compute/automatically-update-instances-in-an-amazon-ecs-cluster-using-the-ami-id-parameter/

[mskrajnowski]

AWS added ASG Instance Refresh which might help with this
https://aws.amazon.com/blogs/compute/introducing-instance-refresh-for-ec2-auto-scaling/
https://docs.aws.amazon.com/autoscaling/ec2/userguide/asg-instance-refresh.html