update terraform EC2 AMI lookup to latest

[diemonster]

As part of a Layer0 instance, we poll which AMIs are available for ECS services during l0-setup apply.

We are currently matching for specific service AMIs here: https://github.com/quintilesims/layer0/blob/500-system-tests-tlake/setup/module/api/core.tf#L167

We should instead use the latest ECS-optimized AMIs available, as we're building more bleeding-edge features. We should probably use a more general filter and the most_recent param ala here: https://www.terraform.io/docs/providers/aws/d/ami.html#description