Adding CLI tests for updating a User's login.

We did not have any tests for updating a User's login via the CLI, but
that may be because you were not allowed to do that before.

```bash
$ nosetests -c robottelo.properties -m test_positive_update_user_2 tests/foreman/cli/test_user.py
2014-05-21 14:38:51 - robottelo - INFO - Paramiko instance prepared (and would be reused): 0x10e875d10
2014-05-21 14:38:55 - root - DEBUG - #1070730 NEW        - satellite6-bugs - User email validation doesn't exactly match specification
2014-05-21 14:38:55 - root - DEBUG - #1070730 NEW        - satellite6-bugs - User email validation doesn't exactly match specification
@Test: Update User with all Compute Resources ... SKIP: Test not implemented
@Test: Update User with one Host group ... SKIP: Test not implemented
@Test: Update User with multiple Host groups ... SKIP: Test not implemented
@Test: Update User with all Host groups ... SKIP: Test not implemented
@Test: Assign a User to an Org ... SKIP: Test not implemented
@Test: Assign a User to multiple Orgs ... SKIP: Test not implemented
@Test: Assign a User to all available Orgs ... SKIP: Test not implemented
@Test: Update User with a new Fact filter ... SKIP: Test not implemented
@Test: Update Login in User ... ok
@Test: Update Login in User ... ok
@Test: Update Login in User ... ok
@Test: Update Login in User ... ok
@Test: Update Login in User ... ok
@Test: Update Login in User ... ok

----------------------------------------------------------------------
XML: foreman-results.xml
----------------------------------------------------------------------
Ran 14 tests in 178.909s

OK (SKIP=8)
```
Merge pull request #849 from omaciel/update-user-login-cli

Adding CLI tests for updating a User's login.