Use IAM user resource instead of hard-coding principals
When attempting to apply !819 (merged), the API returned an error:
Error: Error applying plan: 1 error(s) occurred: * aws_iam_policy.datateam-greenhouse-extract: 1 error(s) occurred: * aws_iam_policy.datateam-greenhouse-extract: Error creating IAM policy datateam-greenhouse-extract: MalformedPolicyDocument: Policy document should not specify a principal. status code: 400, request id: b36409b0-8e56-11e9-95c8-a3b76edfb0bd
This changes to defining the AWS user account within terraform, and uses the
aws_iam_user_policy_attachment resource to attach the previously created policy to that user.