Support Subnets that are configured by external RAs
The IPv6 attributes for subnets provides support for IPv6 subnets that
are managed by non-OpenStack gateway devices, by creating a subnet that
has the ipv6_ra_mode attribute not set, and the ipv6_address_mode
attribute set to IPV6_SLAAC.
In order to support stateless IPv6, Neutron should calculate an IPv6
address based on the IPv6 prefix and MAC address via EUI-64 specification,
and assign this address to the port.