'TestKMSIntegration.test_upload_new_blob_w_bucket_cmek_enabled' flakes w/ EC error

[tseaver]

From this Kokoro build failure:

________ TestKMSIntegration.test_upload_new_blob_w_bucket_cmek_enabled _________

self = <test_system.TestKMSIntegration testMethod=test_upload_new_blob_w_bucket_cmek_enabled>

    def test_upload_new_blob_w_bucket_cmek_enabled(self):
        blob_name = "test-blob"
        payload = b"DEADBEEF"
        alt_payload = b"NEWDEADBEEF"

        kms_key_name = self._kms_key_name()
        self.bucket.default_kms_key_name = kms_key_name
        self.bucket.patch()
        self.assertEqual(self.bucket.default_kms_key_name, kms_key_name)

        blob = self.bucket.blob(blob_name)
        blob.upload_from_string(payload)
        # We don't know the current version of the key.
>       self.assertTrue(blob.kms_key_name.startswith(kms_key_name))
E       AttributeError: 'NoneType' object has no attribute 'startswith'

tests/system/test_system.py:2029: AttributeError

Similar to #190. In this case, reloading the blob after a failure would likely catch up to the expected state.