rally task start scenarios/keystone/create-and-list-users.json
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 Preparing input task
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Input task is:
{
    "KeystoneBasic.create_and_list_users": [
        {
            "args": {},
            "runner": {
                "type": "constant",
                "times": 100,
                "concurrency": 10
            }
        }
    ]
}

Task syntax is correct :)
2016-01-05 18:16:30.005 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Task validation.
2016-01-05 18:16:30.023 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Task validation of scenarios names.
2016-01-05 18:16:30.028 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Completed: Task validation of scenarios names.
2016-01-05 18:16:30.029 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Task validation of syntax.
2016-01-05 18:16:30.033 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Completed: Task validation of syntax.
2016-01-05 18:16:30.034 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Task validation of semantic.
2016-01-05 18:16:30.034 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Task validation check cloud.
2016-01-05 18:16:30.382 23782 INFO rally.task.engine [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Completed: Task validation check cloud.
2016-01-05 18:16:30.400 23782 INFO rally.plugins.openstack.context.keystone.users [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Enter context: `users`
2016-01-05 18:16:30.838 23782 WARNING rally.common.broker [-] Failed to consume a task from the queue: You are not authorized to perform the requested action: identity:get_domain (HTTP 403)
2016-01-05 18:16:30.841 23782 INFO rally.plugins.openstack.context.keystone.users [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Starting:  Exit context: `users`
2016-01-05 18:16:31.850 23782 INFO rally.plugins.openstack.context.keystone.users [-] Task b6aa6a18-3baa-4a82-9173-a147dced6328 | Completed: Exit context: `users`
Task config is invalid: `Unable to setup context 'users': 'Failed to create the requested number of tenants.'`
(rally)jenkins@testvm01:~/test$ rally task start scenarios/keystone/create-and-list-users.json