[AIRFLOW-3801] Fix DagBag collect dags invocation to prevent examples…

[Patouche]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-3801

Description

• Fix in DagBag init method theinclude_examples parameters which was not use properly.

Tests

• tests.models.DagBagTest#test_dont_load_example : When the parameter include_examples is set to false on a empty dags folder, this validate that the dagbag is empty.

Documentation

• N/A

Code Quality

• Passes flake8

[feng-tao]

Could you create the pr against master branch?

[feng-tao]

@Patouche Ping, I think this issue is worth fixing. Let me know if you need any help to continue the pr.