Hi, this PR adds support for ECS task role credentials (see https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-iam-roles.html) with --ecs-creds option or ecs-import-creds helper.

I modified get_key_value function to use sed for improvised JSON parsing, because it couldn't handle non pretty-printed JSON responses. I tested the sed variant for all cases (compact, pretty-printed, with space after colon). If the value doesn't contain double quotes, than it will work well (this would break even the original grep+cut variant).

There are some other minor changes for consistency.

Thank you!