Port "old" API tests for activation keys

Make module `test_activationkey_v2` accomplish everything that module
`test_activationkey` formerly accomplished. Specifically, this means:

* Add method `test_positive_create_4`: "Create an activation key and provide a
  description."
* Update method `test_positive_update_1`. Simplify the test and make sure that
* it updates an activation key's description.

Remove module `test_activationkey`.