Metadata auth

could you please take a look at this one, its for this change
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-authentication.html

example
https://s3.amazonaws.com/cloudformation-templates-us-east-1/S3Bucket_SourceAuth.template

I'm not sure its the best approach tough so if you can share your comments I'm willing to help with this one its blocking us cause we are using this option so I'm currently using this

Since this is, for now, specific to AuthenticationBlock, maybe make it a method or class method of that?  validate_type is a pretty generic function name for such a specific purpose.  Another option is to name it a bit more strictly - validate_authentication_type maybe?