Support Router Advertisement Daemon (radvd) for IPv6

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>
Imported Translations from Transifex

Change-Id: I7e52fc97c41e35b294e464f75351dbc18f5123af
Fix spelling mistake in the log message

Change-Id: I749308b94055eaca0503053daa87f4e89c873c56
Merge "Fix spelling mistake in the log message"
Merge "Support Router Advertisement Daemon (radvd) for IPv6"