Adding basic test for User creation via API.

The following changes add a basic test for the creation of Users via the
API layer. More tests will be added in the near future.
By default, `StringField`s have a `max_len` of 80 characters. You can read the relevant docstring [here](https://github.com/omaciel/robottelo/blob/master/robottelo/orm.py#L87-L89). Are these fields shorter because of some Foreman restriction?