Fixes #6879 - External user groups CLI

Not finished, depends on https://github.com/theforeman/foreman/pull/1640
on my test instance I have `:auth_source_external` in the results, I assume that the field name changes with the source which is unlucky. If this is the case I'd prefer to fix it in the foreman. Otherwise you'd probably have to preprocess the fetched data and unify the field name in hammer.
```
     "results" => [
        [0] {
                              "id" => 1,
                            "name" => "viewers",
            "auth_source_external" => {
                  "id" => 3,
                "type" => "AuthSourceExternal",
                "name" => "External"
            }
        }
    ]
```