Updates metatests docstrings dynamically.

When a test module uses the new metatests, the docstring displayed via
nose are too generic and don't really tells us which Foreman objects are
being tested. The proposed change will dynamically update the generic
docstrings so that the name of the Foreman object is displayed in the
output.

From:

```bash
Fails to creates object. ... ok
Fails to update object shortly after its creation. ... ok
Fails to update object after its creation. ... ok
Successfully creates object. ... ok
Successfully deletes object. ... ok
Successfully updates object. ... ok
```

To:

```bash
Fails to creates object Model. ... ok
Fails to update object Model shortly after its creation. ... ok
Fails to update object Model after its creation. ... ok
Successfully creates object Model. ... ok
Successfully deletes object Model. ... ok
Successfully updates object Model. ... ok
```
Merge pull request #334 from omaciel/update-metatests-docstring

Updates metatests docstrings dynamically.