Support BP:ipv6-router in Neutron HA Router

Support BP:ipv6-router in Neutron HA Router

blueprint ipv6-router (ChangeID:Iaefa95f788053ded9fc9c7ff6845c3030c6fd6df),
supports an IPv6 Router where the router gateway port has no subnet.

The BP implements the following. If an external network (without any subnet)
is attached to the Neutron router, it reads the ipv6_gateway config parameter
(LLA of upstream router) from l3_agent.ini file and adds a default route that
points to this LLA.  If the ipv6_gateway config value is not configured, it
would configure the gateway interface to accept router advts from upstream
router to build the default route.

For an HA router, we would have to configure keepalived to perform this
operation. This patch extends the functionality to an HA router.

Implements: blueprint ipv6-router
Change-Id: I26dc5ce9e46c74423358aa8a9559bc6c7cbdf85e