RDS IAM flow function aws_user_add_to_rds

[sckott]

I was trying to get aws_user_add_to_rds working to make the RDS IAM flow workable, but its still not working yet. AFAIC remember I still wasn't getting the code working correctly for granting a user permissions in the database via IAM in the function. i.e., with the AWSAuthenticationPlugin bit in the code

work is on branch rds-iam-flow