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.
`instance.system_type` looks better than `instance.type_`. Also I think here like `id` we can leave type as is because the `type` builtin will not be override inside a method just in the class definition namespace:

```python
>>> class A(object):
...     type = 10
...     def print_type(self):
...         print type, self.type
...
>>> a = A()
>>> a.print_type()
<type 'type'> 10
```