Test to associate custom and RH contents with activation-key

- Test to associate both custom and RH contents with activation-key
- Updated test stub 'test_end_to_end_activation_key' to accomodate a new scenario
There are some recent changes related to the return values of `entities.Repository(id=repo_id).sync()`
It no longer returns the task_id, instead now it returns the `foreman task_info` which is a dict.

so we need to change the above two statements to:

``` python
task_result = entities.Repository(id=repo_id).sync()['result']
```