Ability to add placement constraints to ecs task definitions

[johnzac]

Would be great if we had the ability to specify placement constraints during task definition registration. (https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-placement-constraints.html) I've added the functionality on our fork of the repo. Would be happy to raise a PR if this is a feature that would make sense.

[josmo]

Thanks @johnzac totally happy to accept a PR :)