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.