manage: Do not use set_defaults on parent parsers with py2

Doing so with cpython < 2.7.9 results in the following bug being hit:

argparse set_defaults on subcommands should override top level set_defaults
https://bugs.python.org/issue9351

As CentOS 7 continues to ship with cpython 2.7.5 this change simply
removes the parent parser call to set_defaults for py2 while leaving the
associated tests unchanged. This should ensure that the actual behaviour
of placement-manage is unchanged while allowing these tests to pass once
again on CentOS.

Change-Id: I87989ced31b1d39e30c992b34fe2ae6db06f7883
Closes-Bug: #1804420

placement/cmd/manage.py

@@ -10,6 +10,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import six
 import sys
 
 from oslo_config import cfg
@@ -43,7 +44,9 @@ def add_db_command_parsers(subparsers):
     # help text.
     subparsers.required = False
     parser = subparsers.add_parser('db')
-    parser.set_defaults(func=parser.print_help)
+    # Avoid https://bugs.python.org/issue9351 with cpython < 2.7.9
+    if not six.PY2:
+        parser.set_defaults(func=parser.print_help)
     db_parser = parser.add_subparsers(description='database commands')
 
     help = _('Sync the datatabse to the current version.')