Making sure bucket delete does not fail in regression tests.

[dhermes]

Adds a safe_delete() which respects objects which are still listed for a bucket but have already been deleted. (This is because the object list is eventually consistent and we delete some objects in our regression tests.)

Fixes #531. See #535 for context.

Fix based on comment from @thobrla.

This is a "temporary" stop-gap and the safe delete can likely be folded into Connection.delete_bucket(). For now, we want to eliminate flaky regression test failures.

[coveralls]

Coverage remained the same at 100.0% when pulling 0b90362 on dhermes:storage-safe-teardown into 927c534 on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0b90362 on dhermes:storage-safe-teardown into 927c534 on GoogleCloudPlatform:master.

[tseaver]

LGTM

[dhermes]

Hilariously, this regression test failed due to flakiness:
https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/47970589

======================================================================
FAIL: test_save_key_self_reference (datastore.TestDatastoreSaveKeys)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/GoogleCloudPlatform/gcloud-python/regression/datastore.py", line 158, in test_save_key_self_reference
    self.assertEqual(len(stored_persons), 1)
AssertionError: 0 != 1

I "fixed" this in #562 but it seems I didn't fully, FULLY understand the eventual consistency issue.