Support ECS Task credentials

[Castaglia]

There will be cases when mod_aws is run from within a Docker container, on AWS ECS. As such, a very nice complement to the existing functionality for obtaining AWS credentials dynamically via IAM instance profile would be to also check for credentials via EC2 Task credentials.

ECS container specific roles (Task roles) are at:

http://169.254.170.2$AWS_CONTAINER_CREDENTIALS_RELATIVE_URI