magnum-api not working with www_authenticate_uri

For magnum 7.0.1 package (Rockyrelease) the keystone_auth section is inconsistent to the keystone mainstream config

When configuring in /etc/magnum/magnum.conf

auth_uri = https://controller:5000/v3

then the /var/log/magnum-api.log contains a warning

WARNING oslo_config.cfg [-] Deprecated: Option "auth_uri" from group "keystone_authtoken" is deprecated for removal (The auth_uri option is deprecated in favor of www_authenticate_uri and will be removed in the S release.). Its value may be silently ignored in the future.

magnum related openstack cli commands properly work, eg

# openstack coe cluster list
+--------------------------------------+---------------+---------+------------+--------------+---------------+
| uuid | name | keypair | node_count | master_count | status |
+--------------------------------------+---------------+---------+------------+--------------+---------------+
| ae2395b8-8616-4721-b5de-9134472496cb | swarm-cluster | None | 1 | 1 | CREATE_FAILED |
| 5d6651a0-7be4-42ff-ad28-bd93633761c2 | swarm-cluster | None | 1 | 1 | CREATE_FAILED |
+--------------------------------------+---------------+---------+------------+--------------+---------------+

so far so good

BUT: when you configure

www_authenticate_uri = https://controller:5000/v3

(as it should be indeed latest since pike)

Then any kind of openstack cli command with magnum elements fails with

'NoneType' object has no attribute 'replace' (HTTP 500) (Request-ID: req-e6701428-5395-40c4-9b64-f0d4765501e9)

/var/log/magnum/magnum-api.log shows

2018-09-21 21:12:43.436 14212 INFO magnum.cmd.api [-] Starting server in PID 14212
2018-09-21 21:12:43.442 14212 INFO magnum.cmd.api [-] Serving on http://10.0.88.11:9511
2018-09-21 21:12:43.442 14212 INFO magnum.cmd.api [-] Server will handle each request in a new process up to 4 concurrent processes
2018-09-21 21:12:43.443 14212 INFO werkzeug [-] * Running on http://10.0.88.11:9511/ (Press CTRL+C to quit)
2018-09-21 21:13:04.193 14287 ERROR wsme.api [req-dc0a815a-822b-4459-95e6-0c5550670155 - - - - -] Server-side error: "'NoneType' object has no attribute 'replace'". Detail:
Traceback (most recent call last):

  File "/usr/lib/python2.7/dist-packages/wsmeext/pecan.py", line 85, in callfunction
    result = f(self, *args, **kwargs)

  File "/usr/lib/python2.7/dist-packages/magnum/api/controllers/v1/cluster.py", line 329, in get_all
    action='cluster:get_all')

  File "/usr/lib/python2.7/dist-packages/magnum/common/policy.py", line 100, in enforce
    add_policy_attributes(target)

  File "/usr/lib/python2.7/dist-packages/magnum/common/policy.py", line 110, in add_policy_attributes
    trustee_domain_id = admin_osc.keystone().trustee_domain_id

  File "/usr/lib/python2.7/dist-packages/magnum/common/keystone.py", line 179, in trustee_domain_id
    access = self.domain_admin_auth.get_access(

  File "/usr/lib/python2.7/dist-packages/magnum/common/keystone.py", line 145, in domain_admin_auth
    auth_url=self.auth_url,

  File "/usr/lib/python2.7/dist-packages/magnum/common/keystone.py", line 47, in auth_url
    return CONF[ksconf.CFG_LEGACY_GROUP].auth_uri.replace('v2.0', 'v3')

AttributeError: 'NoneType' object has no attribute 'replace'

Note: the entire magnum configuration docs are outdated, the still contain description for port 35357 for keystone