Allow some fields to be None

Container, disk format and checksum - among other fields - are not
required parameters, therfore this patch allows them to be None.

This will allow the client to have attributes with value `None` and
it'll make it consistent across responses and with regards to the API.

NOTE: A follow-up patch will enable None values to be returned by the API.

Closes-bug: #1398313

Change-Id: I75da1e9309e0f7ef8839dea3ec9c99c58edc5d63
Merge "Allow some fields to be None"