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

[jathanism]

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.)