l3 notifer should not be a set
l3 notifer should not be a set

An exceptions is raised when a gateway is set to a router because
l3 notifer is a set

Close-Bug: #1793633
Change-Id: I9ae83a9890fb3db2db6150d52a77e67435869770

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow