Deprecation Warning: Airflow Home env var

[bitc0m]

Small change required to the env var airflow_home should be removed from the config and only use AIRFLOW_HOME environment variable.

/usr/local/lib/airflow/airflow/configuration.py:569: DeprecationWarning: Specifying both AIRFLOW_HOME environment variable and airflow_home in the config file is deprecated. Please use only the AIRFLOW_HOME environment variable and remove the config file entry.