DB, IPAM & RPC changes for IPv6 Prefix Delegation

DB, IPAM & RPC changes for IPv6 Prefix Delegation

This patch includes the DB, IPAM & RPC changes needed for the IPv6 Prefix
Delegation feature.

To enable this feature, the subnetpool_id attribute of subnets has been
modified to allow for a special subnetpool identifier - "prefix_delegation".

WORKFLOW:

1. Admin sets default_ipv6_subnet_pool in neutron.conf to "prefix_delegation"
2. User creates a new IPv6 subnet without a CIDR or subnetpool ID
3. User creates an interface between this subnet and a router with an existing
external interface

The agent-side changes will follow in separate patches.

A documentation patch 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

Change-Id: Ic0c6ed4dba74da94a75838178a1837f93d2d0885
Co-Authored-By: Baodong (Robert) Li <baoli@cisco.com>
Partially-Implements: blueprint ipv6-prefix-delegation