Don't retry failures because of execution timeouts

**Description** When reaching execution_timeout, a task fails but continues to retry. Either it should not be retried in this situation, or there should be a possibility to define another timeout for the "total" task execution, taking into account all retries. **Use case / motivation** In our case current behavior makes execution_timeout feature useless: we have retries in place to prevent random issues like network connectivity. At the same time, we want to make sure that the tasks don't run for too long and execution_time + retries would just make them running even longer. See also https://stackoverflow.com/questions/53830604/airflow-execution-timeout-resetting-every-retry: one more request for the same feature.