Add a better description for notification_driver
Add a better description for notification_driver

Improve description for the notification_driver
property in neutron.conf, so users will understand
how to configure it correctly.

Change-Id: Ib5d4c4b6eea5cabf58708d8201f9ff9c883b6c97

bigswitch/neutron,huntxu/neutron,igor-toga/local-snat,mahak/neutron,wolverineav/neutron,sebrandon1/neutron,openstack/neutron,igor-toga/local-snat,bigswitch/neutron,cloudbase/neutron,sebrandon1/neutron,noironetworks/neutron,eayunstack/neutron,klmitch/neutron,eayunstack/neutron,chitr/neutron,chitr/neutron,cloudbase/neutron,apporc/neutron,noironetworks/neutron,dims/neutron,openstack/neutron,klmitch/neutron,MaximNevrov/neutron,mahak/neutron,mahak/neutron,MaximNevrov/neutron,huntxu/neutron,apporc/neutron,openstack/neutron,wolverineav/neutron,dims/neutron