'test_get_signed_policy_v4' flakes with 500

[tseaver]

From this Kokoro build:

_________________ TestV4POSTPolicies.test_get_signed_policy_v4 _________________

self = <test_system.TestV4POSTPolicies testMethod=test_get_signed_policy_v4>

    def test_get_signed_policy_v4(self):
        bucket_name = "post_policy" + unique_resource_id("-")
        self.assertRaises(exceptions.NotFound, Config.CLIENT.get_bucket, bucket_name)
        retry_429_503(Config.CLIENT.create_bucket)(bucket_name)
        self.case_buckets_to_delete.append(bucket_name)

        blob_name = "post_policy_obj.txt"
        with open(blob_name, "w") as f:
            f.write("DEADBEEF")

        policy = Config.CLIENT.generate_signed_post_policy_v4(
            bucket_name,
            blob_name,
            conditions=[
                {"bucket": bucket_name},
                ["starts-with", "$Content-Type", "text/pla"],
            ],
            expiration=datetime.datetime.now() + datetime.timedelta(hours=1),
            fields={"content-type": "text/plain"},
        )
        with open(blob_name, "r") as f:
            files = {"file": (blob_name, f)}
            response = requests.post(policy["url"], data=policy["fields"], files=files)

        os.remove(blob_name)
>       self.assertEqual(response.status_code, 204)
E       AssertionError: 500 != 204

tests/system/test_system.py:2347: AssertionError