[AIRFLOW-1706] Fix query error for MSSQL backend

MSSQL doesn't support key word 'is' as synonym for
'='

Closes apache#2733 from
patsak/fix/illegal_query_for_mssql

Author: k.privezentsev

airflow/jobs.py

@@ -241,7 +241,7 @@ def reset_state_for_orphaned_tasks(self, filter_by_dag_run=None, session=None):
                         TI.execution_date == DR.execution_date))
                 .filter(
                     DR.state == State.RUNNING,
-                    DR.external_trigger.is_(False),
+                    DR.external_trigger == False,
                     DR.run_id.notlike(BackfillJob.ID_PREFIX + '%'),
                     TI.state.in_(resettable_states))).all()
         else: