Arvados

Starting at build 2533, the test failed four times in a row like this:

======================================================================
ERROR: test_KeepSingleCopyRWTest (tests.test_keep_client.KeepTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/tests/test_keep_client.py", line 74, in test_KeepSingleCopyRWTest
    blob_locator = self.keep_client.put(blob_str, copies=1)
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/retry.py", line 153, in num_retries_setter
    return orig_func(self, *args, **kwargs)
  File "/data/1/jenkins/workspace/arvados-api-server/sdk/python/arvados/keep.py", line 1065, in put
    data_hash, copies, thread_limiter.done()), service_errors, label="service")
KeepWriteError: failed to write c902006bc98a3eb4a3663b65ab4a6fab (wanted 1 copies but wrote 0): service http://localhost:44122/ responded with 503 HTTP/1.1 100 Continue
  HTTP/1.1 503 Service Unavailable; service http://localhost:48299/ responded with 503 HTTP/1.1 100 Continue
  HTTP/1.1 503 Service Unavailable

The previous build passed, and this test run was the nightly run with no additional commits, so there was no code change directly responsible. Something else is up. But it's happening reliably enough to be an issue.