Support use of IAM roles on EC2 instance to avoid passing credentials

The key and secret are currently hard requirements, but AWS also supports configuring an EC2 instance to be trusted automatically. Here's how it is done with the official AWS SDK for node:

http://docs.aws.amazon.com/AWSJavaScriptSDK/guide/node-configuring.html

It would be helpful to have this feature in knox as well.

Start with the linked AWS SDK for Node configuration guide, then locate Knox's credential setup and the entry point that currently requires a key and secret. Confirm how credentials are selected and define done as allowing EC2 IAM role credentials without explicitly passed credentials, with coverage for that configuration.

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