Remove action-args from nova-manage help

During help [action_args [action_args ...]] could be
confusing for users

$ nova-manage network list -h
usage: nova-manage network list [-h]
         [action_args [action_args ...]]

Because of that action_args is set to "SUPPRESS" from the help

Change-Id: I0fdc480724d3b59a8eebb2d294f324441c5e46b2
Closes-Bug: #1260538
Merge "Remove action-args from nova-manage help"