Enable ndisc_notify sysctl setting to notify of MAC changes
Enable ndisc_notify sysctl setting to notify of MAC changes

On RHEL/CentOS systems, ndisc_notify is disabled by default. When
OVS restarts or an internal port flaps, the MAC address may change.
Without ndisc_notify, neighbor hosts on the same network will not
know about the MAC change, and will lose connectivity until the MAC
timer expires.

This change sets net.ipv6.conf.all.ndisc_notify to 1, which will
cause a gratuitous neighbor discovery packet which will update MAC
address tables on neighboring hosts.

Change-Id: I89062275541d7c6bb6fb725f5283ba59feb38a94
Closes-bug: 1827927

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