Fix undesirable raw Python error

Using the manilaclient without a subcommand while
passing an optional argument triggers the raw Python
error `ERROR: 'Namespace' object has no attribute
'func'`. This bug can be reproduced by issuing the
command `manila --os-cache`.
Added a default value to `func` and an empty value to
`command` as placeholders so that a help message is
shown instead of the Python error.
This patch uses the same fix implemented in the
python-cinderclient.

Closes-Bug: #1902873
Change-Id: I5fcafd4bdfb40b2054e1205c14493c2f4be1f77a

manilaclient/shell.py

@@ -347,6 +347,9 @@ class OpenStackManilaShell(object):
         parser.add_argument('--os_cert',
                             help=argparse.SUPPRESS)
 
+        parser.set_defaults(func=self.do_help)
+        parser.set_defaults(command='')
+
         return parser
 
     def get_subcommand_parser(self, version):

releasenotes/notes/bug-1902873-fix-py-raw-error-msg-a839fee2ac7b9d3d.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    `Bug #1902873 <https://bugs.launchpad.net/python-manilaclient/+bug/1902873>`_:
+    Fixed raw Python error message when using ``manila`` without
+    a subcommand while passing an optional argument, such as
+    ``--os-cache``.