Support for AWS Elasticsearch using AWS SDK v2

[rosspatil]

Problem: Lacking AWS IAM support to authenticate with Elasticsearch.
Solution: Use AWS SDK v2 to create an IAM session and implement an HTTP round-tripper that will sign requests using Amazon's Signature Verification V4 signing procedure, using credentials from the default AWS credential chain.
To implement this - an extra optional flag is needed to determine the Elasticsearch region i.e. aws.region
if the region is present then only the AWS IAM support will work.