Minimize aws permissions required for ecs/fargate cluster

[jgdwyer]

Intended to address #381

Overview

The goal is to reduce the number of AWS actions/permissions required to run the ECSCluster or FargateCluster. While one can pass skip_cleanup=True to avoid needing many permissions, sometimes one may want cleanup to happen on relevant resources. For example, if one always passes specific iam task roles when launching the cluster, there's no need for the cluster to try to clean up stale iam task roles. By only cleaning up resources that the cluster creates on its own, we can reduce the minimal permissions set.

Changes

• Add conditional statements to only attempt to clean resources if resource arn's have not been provided for them
• Move the _cleanup_stale_resources to be a class method rather than a standalone function
• Break up _cleanup_stale_resources into helper functions for readability purposes

Testing

I've not attempted to test this at all yet, so leaving this as a draft PR for now. But if you all are on board with this kind of approach, I will do some manual testing on my end.