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
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.