Comment 6 for bug 1471316

Me and John had a quick discussion about this, and we came to an agreement on this truth table:

    | CIDR Given in request | Subnet Pool Given in request | Decision that should be made by Neutron
    | Yes | No | CIDR is used, subnet pool = None.
    | Yes | Yes | Neutron will validate CIDR against pool and will either error or suceed accordingly.
    | No | Yes | Subnet pool used is the one specified in the request.
    | No | No | Default subnet pool is used.

Using these cases should provide the most consistant API for the user, and should allow apps that currently call subnet-create without a subnet pool to continue operating as expected.