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
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.