i'm not so keen on this interface. i think once we're in this module, the additional security groups should have already been turned into a list of strings (names)

have a look at 
http://docs.python.org/2.7/library/argparse.html#type
(and possibly https://mail.python.org/pipermail/python-list/2010-September/586458.html) for having argparse help with the conversion