Fix DynamicSelectWidget.get_add_item_url() method.

Do not wrap self.add_item_link_args in an additional list because it
makes impossible to pass more than one arg (i.e. tuple or list of
args) to url resolver.

Change-Id: Idf6d5d1f27c9176f1de93a475e0f1abbae5ee275
Closes-Bug: #1297317
Merge "Fix DynamicSelectWidget.get_add_item_url() method."