Add the default_ipv6_subnet_pool config option

Add the default_ipv6_subnet_pool config option

This config option will allow an admin to set the default subnet-pool
to be used for automatic subnet CIDR allocation in the case of subnet-create
being called without a CIDR or subnet-pool ID.

This also paves the way for enabling IPv6 Prefix Delegation in PD-capable
environments.

DocImpact
Change-Id: Ifdd6e5267e4e5d32e72478ca4ac7e255eb3346ff
Partially-Implements: blueprint subnet-allocation