Implement default subnet pool configuration settings

Implement default subnet pool configuration settings

The default_ipv6_subnet_pool option was added [1] as an integration
point between prefix delegation work and subnet allocation work.  This
patch completes the integration with subnet allocation.  This
addresses the use case where a deployer wants all ipv6 addresses to
come -- by default -- from a globally routable pool of ipv6 addresses.

In a deployment with this option set, an API user can still access the
old behavior by passing None explicitly as subnetpool_id when creating
a subnet.

This patch also adds the default_ipv4_subnet_pool for completeness.

[1] https://review.openstack.org/#/c/166973

Change-Id: I301189b5cd31d7c5fa4a40fa3e04f8e6ac77592b
Partially-Implements: blueprint subnet-allocation