Removes non needed parameter passed in magnum client creation

Magnum client instantiation does not need the user_id parameter
coming from the context to do the operations.

Change-Id: I70070aee03671bf04ba4b933039b2c3fbf07c16f
Closes-Bug: #1786480

mistral/actions/openstack/actions.py

@@ -733,7 +733,6 @@ class MagnumAction(base.OpenStackAction):
             magnum_url=magnum_url,
             auth_token=context.auth_token,
             project_id=context.project_id,
-            user_id=context.user_id,
             auth_url=auth_url,
             insecure=context.insecure
         )