Add notification_driver attribute

This change adds a notification_driver attribute and uses
notification_topics defined in common cookbook.  Changing either will
be reflected in the /etc/neutron/neutron.conf configuration file.

This change will need to merge AFTER
https://review.openstack.org/#/c/76145/ has been merged.

Change-Id: I0e73bed9011b6572425b680ba88c86d1eaed6335