All CLI objects now have class-level methods.

You no longer have to instantiate a CLI object in order to use it. Just
import it and then use its methods directly. These changes won't affect
existing code and/or tests and are therefore back compatible.

```python
In [16]: from robottelo.cli.user import User

In [30]: User.create({'login': 'hsimpson', 'firstname': 'Homer', 'lastname': 'Simpson', 'mail': 'hsimpson@example.com', 'password': 'doh!', 'auth-source-id': 1}).stdout
Out[30]:
{'created-at': '2014/03/03 20:01:23',
 'email': 'hsimpson@example.com',
 'id': '40',
 'last-login': '""',
 'login': 'hsimpson',
 'name': 'Homer Simpson',
 'updated-at': '2014/03/03 20:01:23'}

In [31]: User().create({'login': 'msimpson', 'firstname': 'Marge', 'lastname': 'Simpson', 'mail': 'msimpson@example.com', 'password': 'homie', 'auth-source-id': 1}).stdout
Out[31]:
{'created-at': '2014/03/03 20:02:21',
 'email': 'msimpson@example.com',
 'id': '41',
 'last-login': '""',
 'login': 'msimpson',
 'name': 'Marge Simpson',
 'updated-at': '2014/03/03 20:02:21'}

In [32]: make_user()
Out[32]:
{'admin': None,
 'auth-source-id': 1,
 'created-at': '2014/03/03 20:02:46',
 'email': 'sm5hh6nl@example.com',
 'firstname': '098zz',
 'id': '42',
 'last-login': '""',
 'lastname': '695h97qo',
 'login': 'sm5hh6nl',
 'mail': 'sm5hh6nl@example.com',
 'name': '098zz 695h97qo',
 'password': 'cco2dv',
 'updated-at': '2014/03/03 20:02:46'}
```
Merge pull request #441 from omaciel/all-class-methods-yo

All CLI objects now have class-level methods.