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 the config files as usual and fallbacks to the AWS EC2 metadata service if credentials are not found. We have verified this patch in our cluster.