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

…base_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

defaults/main.yml

@@ -104,7 +104,7 @@ airflow_encrypt_s3_logs: False
 airflow_s3_log_folder: # DEPRECATED
 
 ## DB
-airflow_database_conn: sqlite:////etc/airflow/airflow.db
+airflow_database_conn: sqlite:///{{ airflow_home }}/airflow.db
 airflow_database_pool_size: 5
 airflow_database_pool_recycle: 2000