In addition to Peter H's response... DBInstanceIdentifier is not a supported property. I would consult the cloudformation docs for which properties are and are not supported as well as the required properties. http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-rds-database-instance.html
Also... CloudFormation will tell you the reason it fails in the "Events" tab. One thing CloudFormation is really good at is telling you exactly why it failed.