Neutron oslo_messaging_notifications config error

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

neutron.conf is generated like following:

[oslo_messaging_notifications]
driver = messagingv2
topics =

=> topics value is missing.
This patch fix it.

Closes-Bug: #1671940
Change-Id: I28ab60c61882caaba823bab84f30f77e270f29b4