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.
There is some bit of the code in client.py that does the checking of a hostname that could potentially be moved here. I would suggest to do it in a next pull request, though. Let's keep it simple for now and remove the checking. Thanks!