make 500s change priorities quicker

it's not acceptable for a constantly failing provider to take 50 minutes
to drain (5x reducing priority by 10). But similarly, we need _some_
delay, or a handful of concurrent failures will completely turn off a
provider, rendering the whole excercise kinda pointless. Setting the
delay before it tries to reduce priority again to one minute is nice
because it means that if one request times out and returns 502, then any
other requests that are in flight at that time will time out before the
one minute is up and not switch, but any requests made after the switch
that take sixty seconds to time out will affect it.

tests/app/celery/test_scheduled_tasks.py

@@ -58,7 +58,7 @@ def test_should_call_delete_codes_on_delete_verify_codes_task(notify_db_session,
     assert scheduled_tasks.delete_codes_older_created_more_than_a_day_ago.call_count == 1
 
 
-def test_should_call_delete_invotations_on_delete_invitations_task(notify_api, mocker):
+def test_should_call_delete_invotations_on_delete_invitations_task(notify_db_session, mocker):
     mocker.patch('app.celery.scheduled_tasks.delete_invitations_created_more_than_two_days_ago')
     delete_invitations()
     assert scheduled_tasks.delete_invitations_created_more_than_two_days_ago.call_count == 1
@@ -119,7 +119,7 @@ def test_switch_current_sms_provider_on_slow_delivery_switches_when_one_provider
         created_at=datetime(2017, 5, 1, 13, 50),
         delivery_time=timedelta(minutes=4)
     )
-    mock_reduce.assert_called_once_with('firetext')
+    mock_reduce.assert_called_once_with('firetext', time_threshold=timedelta(minutes=10))
 
 
 @freeze_time('2017-05-01 14:00:00')