Extend method LifecycleEnvironment.path

Client code such as the following can now be written:

```python
>>> LifecycleEnvironment(organization=5).path('organization')
'https://example.com/katello/api/v2/organizations/5/environments'
```

Add tests for this new method. Refactor existing tests so as to reduce the
amount of boilerplate testing code.

Refactor method `ActivationKey.path`, and make its implementation simpler.