Fix subparsers add_parser() regression

With SubCommandOpt, the handler() callback gets invoked with a
subparsers object which has a add_parser() method. The arguments to
this method match the arguments to the ArgumentParser constructor and
the arguments are passed to the class for our ArgumentParser instance.

However, we added a ArgumentParser subclass (_CachedArgumentParser) so
that we could sort the output of --help ... but the subclass doesn't
support all the arguments of the parent class.

glance-control is the only known example of a SubCommandOpt user which
passes unusual arguments to subparsers.add_parser(). It uses the parent
arg and blows up at startup with:

   File "./bin/glance-control", line 276, in add_command_parsers
     parser = subparsers.add_parser(server, parents=[cmd_parser])
   File "/usr/lib64/python2.7/argparse.py", line 1064, in add_parser
     parser = self._parser_class(**kwargs)
 TypeError: __init__() got an unexpected keyword argument 'parents'

It appears we broke glance-control back in June (commit 2951391), but
nobody noticed it until 1.2.0 was released and broke stable/grizzly
because we stopped using glance-control in the Havana glance unit tests
before this regression.

Closes-Bug: #1230416
Change-Id: I8d1b52e5390295726eb49af32eb7cab14c29592c

oslo/config/cfg.py

@@ -1462,8 +1462,8 @@ class _CachedArgumentParser(argparse.ArgumentParser):
     order.
     """
 
-    def __init__(self, prog=None, usage=None):
-        super(_CachedArgumentParser, self).__init__(prog, usage)
+    def __init__(self, prog=None, usage=None, **kwargs):
+        super(_CachedArgumentParser, self).__init__(prog, usage, **kwargs)
         self._args_cache = {}
 
     def add_parser_argument(self, container, *args, **kwargs):

tests/test_cfg.py

@@ -14,6 +14,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import argparse
 import os
 import shutil
 import sys
@@ -2627,6 +2628,21 @@ class SubCommandTestCase(BaseTestCase):
         self.assertEqual(self.conf.cmd.name, 'a')
         self.assertEqual(self.conf.cmd.bar, 10)
 
+    def test_sub_command_with_parent(self):
+        def add_parsers(subparsers):
+            parent = argparse.ArgumentParser(add_help=False)
+            parent.add_argument('bar', type=int)
+            subparsers.add_parser('a', parents=[parent])
+
+        self.conf.register_cli_opt(
+            cfg.SubCommandOpt('cmd', handler=add_parsers))
+        self.assertTrue(hasattr(self.conf, 'cmd'))
+        self.conf(['a', '10'])
+        self.assertTrue(hasattr(self.conf.cmd, 'name'))
+        self.assertTrue(hasattr(self.conf.cmd, 'bar'))
+        self.assertEqual(self.conf.cmd.name, 'a')
+        self.assertEqual(self.conf.cmd.bar, 10)
+
     def test_sub_command_with_dest(self):
         def add_parsers(subparsers):
             subparsers.add_parser('a')