(API) Fix up the "OperatingSystem" entity.

Give the `OperatingSystem` entity three new attributes:

* architecture
* medium
* ptable

Override `OperatingSystem.create` so that these attributes can be correctly
specified. Write new API tests for the "architecture" and "ptable" attributes.
Do not create a test for the "medium" attribute, as it is not currently possible
to create a medium using the API tools.
I dont see a reason why we need two test classes in this module. 
* First test - create os
* second test - create os with architecture
* third test - create os with ptable