Make `ContentView` readable, trim down smoke test

Make `ContentView` inherit from `EntityReadMixin`. use this mixin to trim down
module `tests.foreman.smoke.test_api_smoke`: replace many usages of `client.get`
with `ContentView.read_json`.
`ContentView.read` _almost_ works. The only thing currently blocking it from succeeding is the fact that no `organization_id` attribute is returned when reading a content view, and the `organization` hash which is returned does not contain an ID.