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
Add API tests for host collections

Update the `HostCollection` class definition: add mixins for creation, reading
and deletion. Remove the `system_uuids` attribute: no information of the sort is
provided when reading a host collection, and when creating or updating a host
collection, the attribute seems to conflict with the `system_ids` attribute.

Thoroughly test the relevant API endpoints.
Test ActivationKey/HostCollection associations

Add a test that associates an activation key with zero, one, two and then zero
host collections. Remove a test which currently attempts to test the same thing
but is failing for some unknown reason.