fix: use unique training job names in TensorFlow script mode integ tests

[icywang86rui]

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Description of changes:

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[icywang86rui]

I have run the tests locally the job names look good:

INFO:sagemaker:Creating training-job with name: test-tf-sm-mnist-1554829563-cc7d
INFO:sagemaker:Creating training-job with name: test-tf-sm-mnist-1554829562-9162
INFO:sagemaker:Creating training-job with name: test-tf-sm-distributed-1554829562-4f2f
INFO:sagemaker:Creating training-job with name: test-tf-sm-distributed-1554829563-c1ac

[jesterhazy]

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