(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.
(API) Port old tests for hosts

Significantly update the definition of class `Host`: modify fields and override
the `build` method to properly create dependent entities. Enable CRUD tests for
hosts in module `test_multiple_paths`. Drop old CRUD tests for hosts.

Slightly clean up module `test_host_v2`.