Add 'message' to ContainerActionEvent

[hongbin]

Zun has introduced a new API for container actions and events (https://blueprints.launchpad.net/zun/+spec/container-actions-api). The implementation is provided by the following patch:

* https://review.openstack.org/#/c/524451/ (DB model)
* https://review.openstack.org/#/c/525085/ (DB API and objects)
* https://review.openstack.org/#/c/536260/ (Compute)
* https://review.openstack.org/#/c/548853/ (API layer)
* CLI is pending (https://bugs.launchpad.net/zun/+bug/1753887)

Here is the sample response of this API (https://review.openstack.org/#/c/548853/4/api-ref/source/samples/container-action-get-resp.json):

    {
        "action": "create",
        "events": [
            {
              "event": "container__do_container_start",
              "finish_time": "2018-03-04 17:03:07+00:00",
              "result": "Success",
              "start_time": "2018-03-04 17:02:57+00:00",
              "traceback": null
            }
        ],
        ...
    }

If an exception occurred, the 'result' will be set to 'Error' and the 'traceback' will show the exception's trace. However, we don't have a field to record the exception's message. As a result, this information is lost.

We'd better to improve the container action API to record the exception's message of each event. For example, the response could be as below or something similar:

    {
        "action": "create",
        "events": [
            {
              "event": "container__do_container_start",
              "finish_time": "2018-03-04 17:03:07+00:00",
              "result": "Error",
              "start_time": "2018-03-04 17:02:57+00:00",
              "message": "...",
              "traceback": "...."
            }
        ],
        ...
    }