Fixed #27799 -- Only add 'args' argument once to command parser.

[dracos]

If a custom management LabelCommand or AppCommand uses 'args' then only
the last positional argument provided is passed to the command, because
argparse's behaviour when two calls to add_argument with the same name
is undefined (it appears to put all the arguments bar one in the first
'*' argument, then the last one in the '+' argument).

This shows this with a test, and fixes it by not making the first call
to parser.add_argument('args') if it's a LabelCommand or AppCommand.

[timgraham]

Closing per ticket.