Comment 3 for bug 1514810

This query stands for validating subnet has at least one ip to allocate for dhcp port:

                    range_qry = context.session.query(models_v2.
                        IPAvailabilityRange).join(models_v2.IPAllocationPool)
                    ip_range = range_qry.filter_by(subnet_id=s['id']).first()
                    if not ip_range:
                        raise n_exc.IpAddressGenerationFailure(
                            net_id=cur_subnet.network_id)

But as I see there is no easy way to perform this check for third-party ipam driver.
IPAM interface allows only to allocate and deallocate ips, but info about available ips is not populated over IPAM interface.

This check was added in:
https://review.openstack.org/#/c/174228/

Need to confirm if this check is a MUST or a nice to have one.