L3 agent changes and reference implementation for IPv6 PD

L3 agent changes and reference implementation for IPv6 PD

This patch adds the common framework to be used by specific
implementations of the DHCPv6 protocol for Prefix Delegation.

It also includes a reference implementation based on the Dibbler
DHCPv6 client. Dibbler version 1.0.1 or greater is required.
Sanity tests are included to verify the installed version.

A patch for admin/user documentation is up for review here:

https://review.openstack.org/#/c/178739

Video guides for configuring and using this feature are available on
YouTube:

https://www.youtube.com/watch?v=wI830s881HQ
https://www.youtube.com/watch?v=zfsFyS01Fn0

Co-Authored-By: Baodong (Robert) Li <baoli@cisco.com>
Co-Authored-By: Sam Betts <sam@code-smash.net>
Change-Id: Id94acbbe96c717f68f318b2d715dd9cb9cc7fe4f
Implements: blueprint ipv6-prefix-delegation