Fix config option default value for sample config file

There are multiple places where nova set the new default
value for other (than nova) namespace config option for
example oslo lib config options. Examples:

- https://github.com/openstack/nova/blob/20572809f2d8fefd72a7a059c4e82462a0d66262/nova/config.py#L62
- https://github.com/openstack/nova/blob/20572809f2d8fefd72a7a059c4e82462a0d66262/nova/policy.py#L48

and these defaults are reflected in code also which is working
as expected. But when config sample file is generated via
oslo-config-generator tool (tox -egenconfig) then these defaults
which are set by Nova are not reflected as this tool take the
raw defaults. To solve this issue oslo config provide a option
to add hook to reflect the new default in config generator
- https://docs.openstack.org/oslo.config/latest/cli/generator.html#modifying-defaults-from-other-namespaces

We already doing it for middleware cors option
- https://github.com/openstack/nova/blob/20572809f2d8fefd72a7a059c4e82462a0d66262/setup.cfg#L41

Fixing existing default value and if we find more or any future
change for default value, we need to add those config generator hook.

Change-Id: Ib0e926aec069dfb4ffd1bbfe8506d62464511b51
Closes-Bug: #1904250

setup.cfg

@@ -38,7 +38,7 @@ packages =
 oslo.config.opts =
     nova.conf = nova.conf.opts:list_opts
 oslo.config.opts.defaults =
-    nova.conf = nova.middleware:set_defaults
+    nova.conf = nova.config:set_lib_defaults
 oslo.policy.enforcer =
     nova = nova.policy:get_enforcer
 oslo.policy.policies =