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 not set, and the ipv6_address_mode set to IPV6_SLAAC.

In order to support stateless IPv6, neutron should calculate IPv6
address base on IPv6 prefix and MAC address via EUI-64 specification,
and assign this address to target instance. Also assume the instance
will be assigned with the same IPv6 address

Change-Id: Ia76d76a67e0dc9f996a72e38d987a05aecc1b55f
Co-Authored-By: Xu Han Peng <xuhanp@cn.ibm.com>
Co-Authored-By: Dazhao Yu <dzyu@cn.ibm.com>
When a Subnet's ipv6_ra_mode is None, do not start up dnsmasq

Change-Id: I2d444c2089e87fff7092d5a7bc18b247f126ad0e