Fix Usage of cliff commandmanager

Set namespace to "magnum" when setting up cliff
commandmanager. Earlier None used to work but after
[1] and it's u-c release as cliff 2.10.0([2]) it requires
to be set as some string.

[1] https://review.openstack.org/#/c/522380/
[2] https://review.openstack.org/#/c/531764/

Change-Id: I036386d758a9351f053338f41a595fc030df5a33
Closes-Bug: #1741911

magnum/cmd/driver_manage.py

@@ -89,7 +89,7 @@ class DriverManager(app.App):
         super(DriverManager, self).__init__(
             description='Magnum Driver Manager',
             version=version.version_info,
-            command_manager=DriverCommandManager(None),
+            command_manager=DriverCommandManager('magnum'),
             deferred_help=True)