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`.
Merge pull request #1289 from Ichimonji10/smoke-test

Make `ContentView` readable, trim down smoke test