Replace IAM User and Access Key/Secret Key with IAM Role

Hi Nils,

I'm the Community Manager for AWS CloudFormation and Elastic Beanstalk and saw this repo on speakerdeck. Great presentation and this is a cool application and CloudFormation template!

This PR makes a few small changes to your CloudFormation template to replace the IAM User and Access Key/Secret Key with [IAM Roles](http://docs.aws.amazon.com/IAM/latest/UserGuide/role-usecase-ec2app.html). With the IAM Role there are no longer API credentials in the Instance's UserData or in `/etc/init/photoapp.conf`. As you'll notice, no changes to your app code were required as the AWS SDK for Node.js detects the IAM Role automatically.

Thanks!

Evan