[AIRFLOW-2686] Fix Default Variables not base on default_timezone
[AIRFLOW-2686] Fix Default Variables not base on default_timezone

update func:`TaskInstance.get_template_context` in model.py,
convert datetime variables to default timezone

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