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>
Correct LOG.debug use

The commit b3202c3283597de031572e0ede082237487d8110 used:

  LOG.debug("...%(key1)s...", key1=value1)

which  is not supported, this change ues instead:

  LOG.debug("...%(key1)s...", {"key1": value1})

Change-Id: I9bd63c27ca8acf89284399fda8cd74c82906113b
Merge "Support Router Advertisement Daemon (radvd) for IPv6"
Merge "Correct LOG.debug use"