Code review comment for lp:~blake-rouse/maas/add-osystem-to-node-form-api

One of our architecture decisions is that the API should validate through the forms, so validating in the form should also cover the API.

We normally like our factories to produce randomised fields, not reliable blanks. (If a field can be blank, it can be useful to flip a coin first — heads, leave blank; tails, make up random value.)