Make ActivationKey readable, trim down tests

Make `ActivationKey` inherit from `EntityReadMixin`. Use this mixin to trim down
module `tests.foreman.api.test_activationkey_v2`: replace many usages of
`client.get` with `ActivationKey.read_json`.

All tests pass or are skipped.
Merge pull request #1287 from Ichimonji10/activation-key-test

Make ActivationKey readable, trim down tests
Skip failing test

One of the tests in module `tests.foreman.api.test_contentviews` fails with a
message stating 'System record should be implemented'.

* Systems can now be created via the "new" API layer.
* An existing smoke test does exactly what the failing test is supposed to do.

Stub out this broken test, preventing it from running and failing.
Merge pull request #1288 from Ichimonji10/jenkins

Skip failing test
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.
Merge pull request #1290 from Ichimonji10/test-hostcollection

Add API tests for host collections