Add support for using IAM Roles for SQS queues.

The AWS PHP SDK allows for specifying a location on EC2 instances that will allow the instance to assume an IAM role and connect to different services. You just need to add a `default_cache_config` option to the config array for queues using the SQS driver.
Merge pull request #2819 from andrewryno/aws-iam-roles

Add support for using IAM Roles for SQS queues.