Use service discovery instead of load balancer for internal services

[lukehoban]

With the launch of https://aws.amazon.com/about-aws/whats-new/2018/03/introducing-service-discovery-for-amazon-ecs/, we can now avoid creating load balancer for internal services. This will significantly reduce cost and complexity, and align better with how we’ll want to expose containerized services in Kubernetes.

[lukehoban]

We are adding AWS-specific libraries for working with ECS Services as part of pulumi/pulumi-awsx#53. In the near term, those will offer greater flexibility to control details like this for users who are working with AWS. We'l continue to think about how to add cross-cloud support for this as part of the cloud package.