This method is only used by the cnmallocator to allocate Swarm-scoped
network resources. Its only concrete implementation is in the ovmanager.
Other network drivers are implementing it too to adhere to the
driverapi.Driver interface, but they all return a 'not implemented'
error.
Extract this method into a separate interface, and add a dedicated
RegisterNetworkAllocator to the driver registry. Update the cnmallocator
to load 'network allocators' instead of 'drivers'.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>