BQ system tests: test_create_dataset failed, needs retry

[tseaver]

From https://travis-ci.org/GoogleCloudPlatform/gcloud-python/builds/151011041#L609:

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ERROR: test_create_dataset (bigquery.TestBigQuery)
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Traceback (most recent call last):
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/system_tests/bigquery.py", line 59, in test_create_dataset
 dataset.create()
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/.tox/system-tests/lib/python2.7/site-packages/gcloud/bigquery/dataset.py", line 423, in create
 method='POST', path=path, data=self._build_resource())
 File "/home/travis/build/GoogleCloudPlatform/gcloud-python/.tox/system-tests/lib/python2.7/site-packages/gcloud/connection.py", line 347, in api_request
 error_info=method + ' ' + url)
ServiceUnavailable: 503 Error encountered during execution. Retrying may solve the problem. (POST https://www.googleapis.com/bigquery/v2/projects/precise-truck-742/datasets)
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