ovn: Send GARP on localnet.

ovn: Send GARP on localnet.

In some use cases such as VM migration or when VMs reuse IP addresses, VMs
become unreachable externally because external switches/routers on localnet
have stale port-mac or ARP caches. The problem resolves after some time
when the caches ageout which could be minutes for port-mac bindings or
hours for ARP caches.

To fix this, send some gratuitous ARPs when a logical port on a localnet
datapath gets added. Such gratuitous ARPs help on a best-effort basis to
update the mac-port bindings and ARP caches of external switches and
routers on the localnet.

Reported-at: https://bugs.launchpad.net/networking-ovn/+bug/1545897
Reported-by: Kyle Mestery <mestery@mestery.com>
Signed-off-by: Ramu Ramamurthy <ramu.ramamurthy@us.ibm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>