"node-interface" API should just be "interface" - to allow devices to use it

[Blake Rouse]

The "node-interfaces" and "node-interface" API should really just be "interfaces" and "interface" respectively. This is because it can be used on a device as well, with some limitation for devices.

This will also be important in 2.0 when rack controllers will be able to have there interfaces modified. The same endpoint can be used for that as well.

This will also improve Juju. Juju will be able to use this endpoint on a device to make sure the device gets the correct IP address for the subnet they request. At the moment the "claim-sticky-ip-address" without an address only works for the discovered subnets on the parent. This is fragile and not the best case. The best case is for Juju to tell use exactly what subnet they want and we will claim an IP address for that subnet. Example below of how it would work from Juju.

maas my-session devices new hostname=juju-machine-0-lxc-0 mac_addresses=00:11:22:33:44:55
maas my-session interface link-subnet <new-device-id> <new-device-interface-id> mode=static subnet=<subnet_id>