Make System().create() work

The two commit messages spell out the changes quite clearly. In short:

* Make `System().create()` work.
* Add `System` to some of the test cases in module `tests.foreman.api.test_multiple_paths`.

Note that the `System` entity has not been added to *all* of the tests in `test_multiple_paths`, as the API requires a system's UUID, not ID. This is rather strange, and some code must be devised to adapt to this situation. Overriding `System.path` to use the system's UUID instead of ID may be a good solution.
The API docs don't say much:

> **environment_id**
> optional
> Specify the environment
> Value: String