Use ec2-metadata region endpoint instead of ec2instanceidentity

[jhmartin]

https://github.com/hashicorp/go-discover/blob/master/provider/aws/aws_discover.go#L71-L79 determines the instance region based on the Ec2InstanceIdentityDocument. AWS now offers this as a standalone metadata field. It would be less confusing to fetch the standalone field vs decoding it out of the instance id document.

See:

• https://docs.aws.amazon.com/sdk-for-go/api/aws/ec2metadata/#EC2Metadata.Region aka placement/region
• https://aws.amazon.com/about-aws/whats-new/2020/08/amazon-ec2-instance-metadata-service-supports-additional-fields-improved-automation-operability/