Add exponential backoff for deletion failures.

Fixes: #1657.

Note for potential broader use toward #1619.

system_tests/logging_.py

@@ -50,8 +50,18 @@ def setUp(self):
  self.to_delete = []
 
  def tearDown(self):
+ from gcloud.exceptions import NotFound
  for doomed in self.to_delete:
- doomed.delete()
+ backoff_intervals = [1, 2, 4, 8]
+ while True:
+ try:
+ doomed.delete()
+ break
+ except NotFound:
+ if backoff_intervals:
+ time.sleep(backoff_intervals.pop(0))
+ else:
+ raise
 
  @staticmethod
  def _logger_name():