Fixes --help and command help for cratonclient

Both craton --help and craton help <COMMAND> is
broken for different reasons. This patch fixes
that by adding do_help function for help subcommand
and by displaying suparsed argument of --help.

Closes Bug: 1659092

Change-Id: I838c780ada21edb8202d0fda6bfc982bc57c253b

cratonclient/shell/main.py

@@ -20,6 +20,7 @@ from oslo_utils import encodeutils
 from oslo_utils import importutils
 
 from cratonclient import __version__
+from cratonclient import exceptions as exc
 from cratonclient import session as craton
 
 from cratonclient.common import cliutils
@@ -132,11 +133,17 @@ class CratonShell(object):
         )
         self.parser = subcommand_parser
 
-        if options.help or ('help' in argv) or not argv:
-            parser.print_help()
+        if options.help or not argv:
+            self.parser.print_help()
             return 0
 
         args = subcommand_parser.parse_args(argv)
+
+        # Short-circuit and deal with help right away.
+        if args.func == self.do_help:
+            self.do_help(args)
+            return 0
+
         session = craton.Session(
             username=args.os_username,
             token=args.os_password,
@@ -145,6 +152,22 @@ class CratonShell(object):
         self.cc = client.Client(session, args.craton_url)
         args.func(self.cc, args)
 
+    @cliutils.arg(
+        'command',
+        metavar='<subcommand>',
+        nargs='?',
+        help='Display help for <subcommand>.')
+    def do_help(self, args):
+        """Display help about this program or one of its subcommands."""
+        if args.command:
+            if args.command in self.subcommands:
+                self.subcommands[args.command].print_help()
+            else:
+                raise exc.CommandError("'%s' is not a valid subcommand" %
+                                       args.command)
+        else:
+            self.parser.print_help()
+
 
 def main():
     """Main entry-point for cratonclient's CLI."""