Set arp_notify to match ndisc_notify
Set arp_notify to match ndisc_notify

A previous change [1] enabled ndisc_notify, which is the IPv6 equivalent
for IPv4's arp_notify.  Enable the latter to keep them consistent with
each other.

Change-Id: I15e8348585a9c0ee824a4c123677992010980b9e
Related-Bug: #1827927

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates