FlowETL doesn't pick up custom QA checks

[jc-harrison]

I have tried adding a custom QA check template in '<dag_folder>/qa_checks/', as indicated in the get_qa_checks docstring, but it doesn't get added as a QA check task.

As a workaround, it would be useful to add a template_searchpath parameter to create_dag, so that the path to QA check templates could at least be manually added.

[greenape]

Hmm. Looks like we're trying to include these on the assumption that airflow rolls in the dag automatically to the template env, which it does but at the point of there being a DagRun, not at the point of there being a dag - so when we call dag.template_searchpath as part of our dag creation, the dag folder isn't included.

[greenape]

Looks like we should explicitly add airflow.settings.DAGS_FOLDER to the list we search, but would also make sense to allow adding additional search paths.