Can't create cron trigger when authen with token get from application credentials

[Bo Tran]

When I use application credential, I can't create new cron trigger.
I was perform as follows:
1. Create a new application credential
2. Make a command line auth with above application credential
   OS_AUTH_TYPE=v3applicationcredential
   OS_AUTH_URL=http://127.0.0.1:5000/v3
   OS_IDENTITY_API_VERSION=3
   OS_APPLICATION_CREDENTIAL_ID=17d58fe21c9c49d7a7418bd60b841c73
   OS_APPLICATION_CREDENTIAL_SECRET=qoeIFJztgDVX-LRTWIpQJ-PhzSEh9CBvpxNu-xxxx
3. Create a new cron trigger

I was receiverd a error:
2020-12-20 14:28:23.603 68 ERROR wsme.api [req-31bcbeb3-b925-4c8d-94b0-fbf8e06d861f 3502ac4dda5247829c20b6561c5d188a 4a92824aa74445f28d7cf814a54ee283 - default default] Server-side error: "Authorization failed: Internal Server Error (HTTP 500)". Detail:
Traceback (most recent call last):

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

  File "/opt/mistral/mistral/api/controllers/v2/cron_trigger.py", line 78, in post
    workflow_id=values.get('workflow_id')

  File "/opt/mistral/mistral/db/utils.py", line 106, in decorate
    return retry.call(_with_auth_context, auth_ctx, func, *args, **kw)

  File "/opt/mistral/mistral/utils/rest_utils.py", line 249, in call
    return super(MistralRetrying, self).call(fn, *args, **kwargs)

  File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 391, in call
    do = self.iter(retry_state=retry_state)

  File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 338, in iter
    return fut.result()

  File "/usr/local/lib/python2.7/dist-packages/concurrent/futures/_base.py", line 455, in result
    return self.__get_result()

  File "/usr/local/lib/python2.7/dist-packages/tenacity/__init__.py", line 394, in call
    result = fn(*args, **kwargs)

  File "/opt/mistral/mistral/db/utils.py", line 54, in _with_auth_context
    return func(*args, **kw)

  File "/opt/mistral/mistral/services/triggers.py", line 132, in create_cron_trigger
    security.add_trust_id(trigger_parameters)

  File "/opt/mistral/mistral/services/security.py", line 110, in add_trust_id
    trust = create_trust()

  File "/opt/mistral/mistral/services/security.py", line 41, in create_trust
    client = keystone.client()

  File "/opt/mistral/mistral/utils/openstack/keystone.py", line 41, in client
    auth_url=auth_url

  File "/usr/local/lib/python2.7/dist-packages/keystoneclient/v3/client.py", line 267, in __init__
    self.authenticate()

  File "/usr/local/lib/python2.7/dist-packages/keystoneclient/httpclient.py", line 583, in authenticate
    resp = self.get_raw_token_from_identity_service(**kwargs)

  File "/usr/local/lib/python2.7/dist-packages/keystoneclient/v3/client.py", line 353, in get_raw_token_from_identity_service
    _('Authorization failed: %s') % e)

AuthorizationFailure: Authorization failed: Internal Server Error (HTTP 500)
: AuthorizationFailure: Authorization failed: Internal Server Error (HTTP 500)

This bug have a relationship with https://bugs.launchpad.net/keystone/+bug/1878438 :(