Do not iterate action.choices if it is none

Do not iterate action.choices in the method add_arguments
in the class OpenStackHelpFormatter if action.choices is
not iterable because it is none.

Change-Id: Ie7110adb798326e5856fddfb6a7365c663b84998
Closes-Bug: #1372152
Merge "Do not iterate action.choices if it is none"