Logging: systest teardown flakes with 404.

[tseaver]

From: https://source.cloud.google.com/results/invocations/91034a85-0df7-4b53-bba7-258e166bb3fe/targets/cloud-devrel%2Fclient-libraries%2Fgoogle-cloud-python%2Fpresubmit%2Flogging/log

______________________ TestLogging.test_log_root_handler _______________________
self = <test_system.TestLogging testMethod=test_log_root_handler>
 def tearDown(self):
 retry = RetryErrors((NotFound, TooManyRequests, RetryError), max_tries=9)
 for doomed in self.to_delete:
 try:
> retry(doomed.delete)()
tests/system/test_system.py:111:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../test_utils/test_utils/retry.py:108: in wrapped_function
 return to_wrap(*args, **kwargs)
google/cloud/logging/logger.py:223: in delete
 client.logging_api.logger_delete(self.project, self.name)
google/cloud/logging/_gapic.py:139: in logger_delete
 self._gapic_api.delete_log(path)
google/cloud/logging_v2/gapic/logging_service_v2_client.py:326: in delete_log
 request, retry=retry, timeout=timeout, metadata=metadata
../api_core/google/api_core/gapic_v1/method.py:143: in __call__
 return wrapped_func(*args, **kwargs)
../api_core/google/api_core/retry.py:273: in retry_wrapped_func
 on_error=on_error,
../api_core/google/api_core/retry.py:182: in retry_target
 return target()
../api_core/google/api_core/timeout.py:214: in func_with_timeout
 return func(*args, **kwargs)
../api_core/google/api_core/grpc_helpers.py:59: in error_remapped_callable
 six.raise_from(exceptions.from_grpc_error(exc), exc)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
value = NotFound(u'Log handler_root-1563211829508 does not exist',)
from_value = <_Rendezvous of RPC that terminated with:
	status = StatusCode.NOT_FOUND
	deta...pc_message":"Log handler_root-1563211829508 does not exist","grpc_status":5}"
>
 def raise_from(value, from_value):
> raise value
E NotFound: 404 Log handler_root-1563211829508 does not exist
.nox/system-2-7/lib/python2.7/site-packages/six.py:737: NotFound

Superficially similar to #5632, but in this case, the log being deleted has a unique name.

[tseaver]

PR #8837 should resolve this (better to leak log entries than fail CI).