IPv6 router support

Per comments to https://review.openstack.org/#/c/136878/, contents
related to centralized IPv6 router support is taken out and revised to
create this spec.

Change-Id: I33ef9569cf9302d465d5b8d964fa705034c19aa3
Implements: blueprint ipv6-router-and-dvr
Merge "IPv6 router support"