Basic subnetpool CRUD

Basic subnetpool CRUD

Enable creating, reading, updating, and deleting subnet pools via REST API.
Includes required changes to REST, model, alembic migrations, and unit tests.
Subnet pools carry a list of IPv4 or IPv6 prefixes from which a subnet can be
allocated. This will enable tenants to request a subnet from a pool rather
than being forced to explicitly provide their own CIDR's for their subnets.
This change simply enables managing the lifecycle of a subnet pool and does
not yet enable allocation of subnet prefixes from a pool.

Subnet pools can have their prefix bounds (min, max, default), name, and
prefix list updated. Changes to prefix bounds do not alter existing
allocations and will not be blocked by existing allocations. Prefix lists can
only be appended to. Prefixes cannot be removed from the pool once added.

ApiImpact
Partially-Implements: blueprint subnet-allocation
Change-Id: I88c6b15aab258069758f1a9423d6616ceb4a33c4