[AIRFLOW-3090] Make No tasks to consider for execution debug (#3923)

During normal operation, it is not necessary to see the message.  This
can only be useful when debugging an issue.

Author: wrp

airflow/jobs.py

@@ -1100,7 +1100,7 @@ def _find_executable_task_instances(self, simple_dag_bag, states, session=None):
         task_instances_to_examine = ti_query.all()
 
         if len(task_instances_to_examine) == 0:
-            self.log.info("No tasks to consider for execution.")
+            self.log.debug("No tasks to consider for execution.")
             return executable_tis
 
         # Put one task instance on each line