Implement airflow triggerer

[adwk67]

Currently the airflow-operator does not implement an airflow triggerer process/role. This allows deferrable operators to be used in DAGs so that worker management can be improved e.g. on scheduler re-starts and for providing HA: see here for more details.

Acceptance criteria:

• add an airflow role triggerer
• create a test that uses a DAG/deferrable operator that keeps running over a re-deployment/scheduler re-start