ecs: add capacity providers

[mskrajnowski]

AWS added ECS capacity providers which can control autoscaling groups based on the cluster resource requirements. Would be nice to switch to that instead of manually controlling the desired count of instances.

• https://docs.aws.amazon.com/AmazonECS/latest/developerguide/cluster-capacity-providers.html
• https://docs.aws.amazon.com/AmazonECS/latest/developerguide/cluster-auto-scaling.html#asg-capacity-providers
• https://www.terraform.io/docs/providers/aws/r/ecs_capacity_provider.html