[AIRFLOW-2752] Log using logging instead of stdout
[AIRFLOW-2752] Log using logging instead of stdout

Logging should be done using the Python log module
instead of writing directly to stdout

Closes #3604 from Fokko/patch-2

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow