title: "Dependencies not met" complaint for example given in airflow docs
body: <p>Following the airflow demo tutorial given in the docs (<a href="https://airflow.incubator.apache.org/tutorial.html#recap" rel="nofollow noreferrer">https://airflow.incubator.apache.org/tutorial.html#recap</a>) (just a started using airflow for trial test), I notice that there are initial complaints about 'unmet dependencies' for certain tasks when running backfill <code>airflow backfill tutorial -s 2015-06-01 -e 2015-06-07</code>.</p>
  
  <pre><code>[me@mapr001 dags]$ airflow backfill tutorial -s 2015-06-01 -e 2015-06-07
  [2018-07-20 14:34:29,868] {__init__.py:45} INFO - Using executor SequentialExecutor
  [2018-07-20 14:34:29,917] {models.py:189} INFO - Filling up the DagBag from /home/me/airflow/dags
  [2018-07-20 14:34:30,659] {models.py:1197} INFO - Dependencies all met for &lt;TaskInstance: tutorial.print_date 2015-06-06 00:00:00 [scheduled]&gt;
  [2018-07-20 14:34:30,664] {base_executor.py:49} INFO - Adding to queue: airflow run tutorial print_date 2015-06-06T00:00:00 --local -sd DAGS_FOLDER/tutorial.py
  ......
  [2018-07-20 14:34:30,806] {models.py:1190} INFO - Dependencies not met for &lt;TaskInstance: tutorial.templated 2015-06-01 00:00:00 [scheduled]&gt;, dependency 'Trigger Rule' FAILED: Task's trigger rule 'all_success' requires all upstream tasks to have succeeded, but found 1 non-success(es). upstream_tasks_state={'skipped': 0, 'successes': 0, 'failed': 0, 'upstream_failed': 0, 'done': 0, 'total': 1}, upstream_task_ids=['print_date']
  [2018-07-20 14:34:30,815] {models.py:1190} INFO - Dependencies not met for &lt;TaskInstance: tutorial.templated 2015-06-03 00:00:00 [scheduled]&gt;, dependency 'Trigger Rule' FAILED: Task's trigger rule 'all_success' requires all upstream tasks to have succeeded, but found 1 non-success(es). upstream_tasks_state={'skipped': 0, 'successes': 0, 'failed': 0, 'upstream_failed': 0, 'done': 0, 'total': 1}, upstream_task_ids=['print_date']
  ......
  </code></pre>
  
  <p>This seems to indicate something not good (<a href="https://airflow.incubator.apache.org/concepts.html#trigger-rules" rel="nofollow noreferrer">https://airflow.incubator.apache.org/concepts.html#trigger-rules</a>), yet the backfill appears to end successfully in the terminal and web UI.</p>
  
  <p>So my question is: what is going on here? Is this normal behavior (eg. the 'templated' tasks checked if it could run, saw that the 'print_date' task was not done, so just waited)? Is this explained outright somewhere in the docs?</p>