Nova oslo_messaging_notifications config error

With the following configuration in globals.yml
enable_ceilometer="no"
enable_designate="no"
enable_searchlight="yes"

nova.conf is generated like following:

[oslo_messaging_notifications]
driver = messagingv2
topics =

topics value is missing.

Change-Id: I27145c0da8b864b2614091933c33d83bdec8b9be
Closes-Bug: #1671935
Co-Authored-By: Jeffrey Zhang <jeffrey.zhang@99cloud.net>