Support IPv6 DHCPv6 Stateful mode in dnsmasq

Based on ipv6-two-attributes blueprints, this blueprint tries to cover the following three scenarios:

1) If end user chooses DHCPV6 Stateful as IPv6 RA mode and IPv6 Address mode, via either Dashboard or Neutron CLI, then OpenStack will send out RA to tenant network with A bit set to 0 and LMO bits set to 1. In addition, dnsmasq driven by OpenStack will also act as DHCPv6 Stateful server so tenant VM can use its service for address assignment and optional information. At the same time, VM will continue to rely on RA for default route.

2) If end user chooses DHCPV6 Stateful as IPv6 RA mode and set IPv6 Address mode to OFF, via either Dashboard or Neutron CLI, then OpenStack will send out RA to tenant network with A bit set to 0 and LMO bits set to 1. As the result, VM will continue to rely on RA for default route. However, OpenStack will NOT provide DHCPv6 Stateful server for address assignment and optional information. In this case, we assume that some devices out of OpenStack's control will provide address assignment and optional information.

It is worth mentioning that, the scenario 1) and the scenario 2) are only supported if tenant network is routable, i.e. the corresponding IPv6 subnet is attached to a neutron router and a router port is created for tenant gateway. When tenant subnet is NOT attached to any neutron router, (i.e. does NOT have gateway port), OpenStack should NOT be responsible for sending IPv6 RA.

3) If end user sets IPv6 RA mode to OFF and choose DHCPv6 Stateful as IPv6 Address mode, via either Dashboard or Neutron CLI, then dnsmasq driven by OpenStack will only act as DHCPv6 Stateful server for address assignment and optional information. The dnsmasq process is NOT responsible for sending any RA to tenant network. In this case, we assume that some devices out of OpenStack's control will announce IPv6 RA to tenant network.

It is worth mentioning that, the scenario 3) is only supported if tenant network is private or provider network. In other words, the tenant subnet is NOT attached to any neutron router and does NOT have gateway port on neutron router.