[AIRFLOW-2197] Silence hostname_callable config error message

Since the hostname_callable key is not defined in the config, we end
up with a lot of warnings. Add the key to the config and simplify the
code.

.gitignore

@@ -62,6 +62,7 @@ nosetests.xml
 coverage.xml
 *,cover
 .hypothesis/
+.pytest_cache
 
 # Translations
 *.mo

airflow/config_templates/default_airflow.cfg

@@ -56,9 +56,8 @@ logging_config_class =
 log_format = [%%(asctime)s] {{%%(filename)s:%%(lineno)d}} %%(levelname)s - %%(message)s
 simple_log_format = %%(asctime)s %%(levelname)s - %%(message)s
 
-# Hostname override by providing a path to a callable.
-# hostname_callable = socket:getfqdn
-
+# Hostname by providing a path to a callable, which will resolve the hostname
+hostname_callable = socket:getfqdn
 
 # Default timezone in case supplied date times are naive
 # can be utc (default), system, or any IANA timezone string (e.g. Europe/Amsterdam)

airflow/config_templates/default_test.cfg

@@ -40,6 +40,7 @@ non_pooled_task_slot_count = 128
 enable_xcom_pickling = False
 killed_task_cleanup_time = 5
 secure_mode = False
+hostname_callable = socket:getfqdn
 
 [cli]
 api_client = airflow.api.client.local_client

scripts/ci/airflow_travis.cfg

@@ -25,6 +25,7 @@ dags_are_paused_at_creation = False
 default_impersonation =
 secure_mode = False
 fernet_key = af7CN0q6ag5U3g08IsPsw3K45U7Xa0axgVFhoh-3zB8=
+hostname_callable = socket:getfqdn
 
 [webserver]
 base_url = http://localhost:8080