Migrate issues from files-airflow

[pandemicsyn]

There's a couple of feature and bug reports on files-airflow that we might want to copy/redirect folks from

• Execute parallel tasks for nested tasks in a job spec files-airflow#21
• Scheduling DAGs with @once - not supported files-airflow#22
• Airflow doesn't trigger job because dag's start_date was set to datetime.utcnow() files-airflow#23 (has a contributed PR ready to merge - that will need to be replicated here)

[pandemicsyn]

(merged and brought over meltano/files-airflow#23)

[pandemicsyn]

@aaronsteers also has a pending PR to set --log-level=warning that we should bring over when it lands: -

• fix: quieter logs when polling schedule changes files-airflow#28

[pandemicsyn]

New issue that will need to come over:

• Extend dag generator to inject env for jobs files-airflow#32