[AIRFLOW-2034] Fix mixup between %s and {} when using str.format

Convention is to use .format for string formating oustide logging, else use lazy format
See comment in related issue
https://github.com/apache/incubator-airflow/pull/2823/files
Identified problematic case using following command line
.git/COMMIT_EDITMSG:`grep -r '%s'./* | grep '\.format('`

Closes apache#2976 from knil-sama/fix-mixup-format-str

airflow/hooks/slack_hook.py

@@ -52,5 +52,5 @@ def call(self, method, api_params):
         rc = sc.api_call(method, **api_params)
 
         if not rc['ok']:
-            msg = "Slack API call failed (%s)".format(rc['error'])
+            msg = "Slack API call failed ({})".format(rc['error'])
             raise AirflowException(msg)

airflow/task/task_runner/base_task_runner.py

@@ -96,9 +96,9 @@ def _read_task_logs(self, stream):
                 line = line.decode('utf-8')
             if len(line) == 0:
                 break
-            self.log.info(u'Job {}: Subtask {} %s'.format(
-                self._task_instance.job_id, self._task_instance.task_id),
-                line.rstrip('\n'))
+            self.log.info('Job %s: Subtask %s %s',
+                          self._task_instance.job_id, self._task_instance.task_id,
+                          line.rstrip('\n'))
 
     def run_command(self, run_with, join_args=False):
         """