ovsdb netvirt needs help in getting mac for a given ip in br-ex

in ovsdb net-virt, neutron will provide an ip as the next-hop default gateway
for connections beyond br-ex.

Since ovsdb has no real arp, it needs a service in can subscribe so that the
mac address associated to the gateway ip address is available.

Today, that mac is hard coded via the config param 'ovsdb.l3gateway.mac', provided in custom.properties file.