Tackles kubernetes/autoscaler#1208. EC2 Metadata helpfully supplies AWS
Region info, so rather than _requiring_ an environment variable, this
patch enables `aws_manager.go` to retrieve that information from EC2
itself.
Example YAMLs no longer reference the vestigial `AWS_REGION` environment
variable, but supplying it to `cluster-autoscaler` still relays it to
the AWS SDK like before.