Incorrect handling of nested intrinsic functions in join()

[zhelyan]

e.g.

join("-", split("_", ref('Label')))

yields

An error occurred (ValidationError) when calling the CreateStack operation: 
Template error: every Fn::Join object requires two parameters, (1) a string delimiter 
and (2) a list of strings to be joined or a function that returns a list of strings 
(such as Fn::GetAZs) to be joined.