Right, so "user data must be base64-encoded before being submitted to the API. The API command line tools perform the base64 encoding for you. The data is decoded before being presented to the instance." (http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AESDG-chapter-instancedata.html)

Trying to put myself in the mindset of someone looking at this for the first time... I'd expect USER_METADATA_STRING and USER_METADATA_MAP to result in injecting data in the same way. Given the description saying it is just for AWS, then I'd expect them both to relate to `http://169.254.169.254/latest/meta-data/` - except for the clue in the description that says it's actually "user data" rather than "user metadata" (contrary to the config name that will appear in properties/yaml). 

As for us making it cloud-neutral, I think that is premature generalization that will just confuse people who are familiar with the names on EC2.