Allow Interfaces to be created with null description, speed, or type using defaults

For these `Interface` fields:
- description
- speed
- type

If these fields are presented as `null` or `None` values on incoming payloads on create (`POST`), then the NSoT default value should be used.

This is to simplify client code for fields that cannot have null values, but which _do_ have defaults on the backend. (Currently there is a workaround for this in pynsot CLI util, but we shouldn't need to do that.)