Added support to get AWS creds from IAM Roles

[rberger]

Fixed recipes to work if an IAM Role is being used instead of having credentials in databag in a way that should be backwards compatible.

Added an attribute ebs[:creds][:iam_role] which when set true will not load or use the credentials databags and not set: aws_access_key or aws_secret_access_key in any of the aws_ebs_* resources.

Made the volume recipe start with at least devid >= 'f'. I found that if you didn't have the ephermal disks in use (at least with a t2 instance) it would start at devid == 'b' which would then fail. Added logic to start at f or later.

Updated the README to reflect the changes

Only tested for the volumes recipe though updated all the other recipes that use aws credentials