Also updates github.com/aws/aws-sdk-go and adds github.com/beevik/etree. Fixes terraform-providers/terraform-provider-aws#3129.
Here we upgrade the AWS Go SDK to 1.12.27 and AWS provider to include terraform-providers/terraform-provider-aws#1608. This includes the capability to use named credentials profiles from the `~/.aws/credentials` file to authenticate to the backend.