Fix "Attempt 2 out of 1" messages

[jlowin]

It's possible to get strange "Attempt 2 out of 1" status messages when Airflow forces a task to run after it's completed or previously run.

For example, this unit test in CoreTests prints such a message (https://github.com/airbnb/airflow/blob/aee3cacb1042dbdc26d9ba06d481c81948f418ec/tests/core.py#L467):

    def test_raw_job(self):
        TI = models.TaskInstance
        ti = TI(
            task=self.runme_0, execution_date=DEFAULT_DATE)
        ti.dag = self.dag_bash
        ti.run(force=True)

The "Attempt" message is built as

            tot_tries = task.retries + 1
            msg = "Attempt {} out of {}".format(self.try_number+1,
                                                tot_tries)
            self.try_number += 1

So every time the task gets loaded, the number of attempts is incremented. If the task is run an unexpected number of times, either due to prior failures or force runs, then this message becomes nonsensical. I think this is straightforward to fix by resetting self.try_number any time the task is run that ISN'T a retry.

[jlowin]

Closed by #1230