Change how field name mappings are used

Remove method `robottelo.factories.Factory._customize_field_names`. Push the
functionality of that method out into function
`robottelo.factories._copy_and_update_keys`. Doing this allows for easier
testing and fewer side-effects.

Change how field name mappings are created. Do not create dicts like this:

    api_names = {
        'name': 'model[name]',
        'info': 'model[info]',
    }

Instead, create two-tuples like this:

    api_names = (
        ('name', 'model[name]'),
        ('info', 'model[info]'),
    )