Support service PlacementConstraints

[mmdriley]

With the 2017-04-28 release of CloudFormation, the AWS::ECS::Service resource now supports the PlacementConstraints and PlacementStrategies properties.

PlacementConstraints seems very useful for the case where an N-task service is scheduled on an N-instance cluster. If nodes cycle, ECS will reschedule tasks to healthy instances, but won't move them back when the instances are replaced. This can leave the majority of tasks on a small number of instances, or all grouped in one AZ, which isn't great for availability. If we set PlacementConstraints to distinctInstance we can prevent two tasks from the same job from starting on the same container instance.