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.

Implements blueprint ipv6-provider-nets-slaac

DocImpact

Change-Id: Ia76d76a67e0dc9f996a72e38d987a05aecc1b55f
Co-Authored-By: Xu Han Peng <xuhanp@cn.ibm.com>
Co-Authored-By: Dazhao Yu <dzyu@cn.ibm.com>
Merge "Support Subnets that are configured by external RAs"