Updates to boto to support EC2 IAM roles (security tokens)

EC2 roles are a much more secure way to assign keys and secrets to an instance, and I've updated the elb-presence to support this. Not sure what happens in the case where plain old access keys are used, as I don't use these anywhere in my environment.
You know what, you are totally right, my brain is stuck in `go` land, where `os.Getenv` returns empty strings for empty things. apologies.