Add test for REST API GET/PUT round-trip

[glennmatthews]

Closes: #DNE

What's Changed

Testing idea that came out of a recent code review (I don't recall the specific one) - make sure the REST API is round-trip capable, i.e. that a client can GET an object, make any (or none) desired changes to the returned object, and send it back in via a PUT without needing to do any massaging of the object data to make it acceptable.

I've implemented this as a generic API test case, and fixed a number of issues it uncovered:

• Choices fields are represented in GET response as {value: X, label: Y} but didn't support this format on write.
• Interface.mac_address and VMInterface.mac_address are nullable, but the API wasn't permitting a client to send null values for these fields.
• DeviceType.front_image and DeviceType.rear_image similarly are nullable but were not marked as such in the API.
• Fixed an unrelated warning I saw in the test logs where we were passing timezone-naive datetimes when creating JobResults in one test.
• User.password was incorrectly marked as a required API field on write.

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• n/a Attached Screenshots, Payload Example
• Unit, Integration Tests
• n/a Documentation Updates (when adding/changing features)
• n/a Example Plugin Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design