Support IAM roles in PrestoS3FileSystem

Currently ``PrestoS3FileSystem`` reads AWS credentials from Hadoop configuration and because of that we need to put our AWS keys in core-site.xml. Ideally we don't want to store our keys in any config file on the cluster and we want to use IAM roles instead of keys (http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/emr-iam-roles.html). This PR brings IAM role support to ``PrestoS3FileSystem``. The ``PrestoS3FileSystem`` first checks the credentials in Hadoop configuration as usual and fallbacks to the AWS EC2 metadata service if credentials are not found. We have verified this patch in our cluster.