Fixed_ips update not supported for non-VIF ports

[Brandon Berg]

Neutron allows updates of the fixed_ips field for all port types, whereas the port translator in midonet-cluster supports this only for VIF ports. For the time being we're going to block updates of fixed_ports for non-VIF ports, but eventually we need to update the translator to support this for all port types.

Note that this involves more than just updating the corresponding Midonet port. We'll need to go through the creation logic for all the port types, and also RouterInterface and RouterGateway, to see what we do with the ports' fixed IP allocations and update everything accordingly.