[AIRFLOW-2739] Always read default configuration files as utf-8
[AIRFLOW-2739] Always read default configuration files as utf-8

Closes #3593 from cjgu/airflow-2739

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