Support new IPv6 subnet modes in Neutron

This addresses addition of two new IPv6 subnet modes
called IPv6_RA_mode and IPv6_address_mode options in Neutron
at the time of creating an IPv6 subnet.

There are fixed values for both these modes.
They can be set to no_attribute_value or one of - slaac,
dhcpv6_stateful or dhcpv6_stateless.

These values represent how Router Advertisements (RAs) are
made for IPv6 address Neighbor discovery for VMs.

Co-Authored-By: Akihiro Motoki <motoki@da.jp.nec.com>
Implements: blueprint neutron-subnet-mode-support
Change-Id: I97abd3642b7da94cda85425534d934f30feb86d2
Merge "Support new IPv6 subnet modes in Neutron"