support for EC2 IAM roles with Amazon S3 file store/backup

Adds new option under S3 section of Settings->Files to "use S3 IAM profile". This option is useful for Discourse instances running on Amazon EC2 .If enabled, this will retrieve the access key ID and secret key details from the IAM profile associated with the EC2 instance--for both S3 file storage and backups.

Cf: https://meta.discourse.org/t/allow-use-of-aws-ec2-iam-roles-with-s3-file-image-uploads/17197?u=windhamg1