Code review comment for ~chad.smith/cloud-init:unify-datasource-get-data

@smoser: addressed discussions we had at the Ubuntu Rally about standardizing metadata keys in instance-data.json and versioning it. The branch is getting big (and therefore risky) so I didn't address some cloud-name renames and surfacing 'platform' in this branch as it would involve a bit of renaming in DataSourceEc2.Platforms too and I want to tackle that a separate branch to get fresh eyes on it.