Fix: the "follow" of TFJobClient.get_logs

[Windfarer]

IMO, the follow parameter of TFJobClient.get_logs method should continually fetch and print the log stream of every listening pods. But the actual behavior is we can only get the log after pod's execution is finished.

As the discussion of this kubernetes-client/python#199 the correct way to get the stream is to call read_namespaced_pod_log with watch.Watch.

This PR is an approach to fix this issue.

The output of watch.stream is a generator, so we could iterate over every pods's stream, group log lines and print them.

[aws-kf-ci-bot]

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[coveralls]

Coverage remained the same at 71.479% when pulling d957a6f on Windfarer:watch_log into cadfd91 on kubeflow:master.

[terrytangyuan]

/lgtm

[gaocegege]

/lgtm
/assign @jinchihe

[gaocegege]

/ok-to-test

[jinchihe]

/lgtm
/approve

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jinchihe, terrytangyuan, Windfarer

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