acme/challenge_request handles only 1 domain name

No other functions inside acme/ handle lists, so
to keep the class consistent, challenge_request()
should only handle a string parameter. Iteration
should be moved to the client, which is handling 
the list of domain names anyway.
Sorry, this is my first python project. My intent was to TDD this function and show explicitly in the test that the parameter is required. I think this does that correctly - is there a better (or more py) way to represent this?