defaults/main.yml: Use {{ airflow_home }} in the default airflow_database_conn
defaults/main.yml: Use {{ airflow_home }} in the default airflow_database_conn

Note: this also resloves this sqlite error on default installs on
ubuntu 18.04:

See:
https://stackoverflow.com/questions/18208492/sqlalchemy-exc-operationalerror-operationalerror-unable-to-open-database-file/44687471#44687471
https://linlog.skepticats.com/entries/2018/03/Database_URIs_are_a_pain.php

idealista/airflow-role