Launch radvd from inside l3 agent when any router port has an IPv6 address. If
slaac is used for IPv6 addresses, advertise the prefix associated with the port;
otherwise, advertise default route only.
Change-Id: Ib8b0b3e71f7af9afa769c41357c66f88f4326807
Implements: blueprint neutron-ipv6-radvd-ra
Co-Authored-By: Henry Gessau <gessau@cisco.com>