added test for model update

please review it
thanks
As an example, [this](https://github.com/omaciel/robottelo/blob/master/tests/foreman/api/test_activationkey_v2.py#L368-L374) docstring produces [this](http://robottelo.readthedocs.org/en/latest/api/tests.foreman.api.html#tests.foreman.api.test_activationkey_v2.ActivationKeysTestCase.test_get_releases_content) html output, whereas [this](https://github.com/omaciel/robottelo/blob/master/tests/foreman/cli/test_model.py#L20-L24) docstring produces [this](http://robottelo.readthedocs.org/en/latest/api/tests.foreman.cli.html#tests.foreman.cli.test_model.TestModel.test_create_model_1) HTML output.