YAQL evaluation of event types does not safe

[ITD27M01]

The definitions from "event_definitions.yaml" [1] configuration file can lead to event processing thread crashes without any logs and notifications.

For example in the cases when the context of the event does not have a project attribute, the following definition will crash the thread and stops event processing. The events will be put to the queue but never processed becasue the thread is silently died [3]:

~~~~~~~~~~~~
- event_types:
    - compute.*
  properties:
    payload: <% $.payload %>
    project_id: <% $.context.project %>
    user_id: <% $.context.user %>
~~~~~~~~~~~~

It is difficult to debug such a situation because of the error will be written to the stdout only during service shutdown but not when the issue has occurred. I think it is because of the threading nature of the event processing code.

As a workaround, I am using the get method for attributes:

~~~~~~~~~~~~
- event_types:
    - compute.*
  properties:
    payload: <% $.get('payload') %>
    project_id: <% $.context.get('project') %>
    user_id: <% $.context.get('user') %>
~~~~~~~~~~~~

[1] https://github.com/openstack/mistral/blob/master/etc/event_definitions.yml.sample

[2] https://github.com/openstack/mistral/blob/541aabbfe8a7afd876adcfb209d18aafdef2d5f6/mistral/event_engine/default_event_engine.py#L73

[3] https://gist.github.com/ITD27M01/4330cca676a6fddad7e20f6c364d4832