Fix stack trace on incorrect nova-manage args.

This error reporting needs more work, but this at least stops us
from presenting a stack trace to the user and is as broken as
before.

Resolves bug 1091403.

Change-Id: I941983b60cc3762a916d57b7ea67264e093cfa44

bin/nova-manage

@@ -1131,8 +1131,11 @@ def main():
     try:
         cliutils.validate_args(fn, *fn_args, **fn_kwargs)
     except cliutils.MissingArgs as e:
+        # NOTE(mikal): this isn't the most helpful error message ever. It is
+        # long, and tells you a lot of things you probably don't want to know
+        # if you just got a single arg wrong.
         print fn.__doc__
-        parser.print_help()
+        CONF.print_help()
         print e
         sys.exit(1)
     try: