Deprecate IsDefaultPS in TFJob CRD API

[ScorpioCPH]

This PR fixes #329:

• Remove IsDefaultPS in TFJob CRD API
• Clean up hack/grpc_tensorflow_server/grpc_tensorflow_server.py which is useless
• Update related docs

@jlewi @gaocegege @DjangoPeng PTAL.

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This change is 

[coveralls]

Coverage decreased (-0.6%) to 30.933% when pulling 7864dc8 on ScorpioCPH:deprecate-default-ps into 11b2fad on tensorflow:master.

[ScorpioCPH]

/test tf-k8s-presubmit

[jlewi]

Anybody else want to take a look?

[wbuchwalter]

setDefault_PSPodTemplateSpec is still present in pkg/apis/tensorflow/v1aplha1/defaults.go and not specifying a PodTemplateSpec for the PS is still valid (see`https://github.com/ScorpioCPH/k8s/blob/deprecate-default-ps/pkg/apis/tensorflow/validation/validation.go#L22).

So this would mean that if the user does not specify a custom image for the PS, there would be no error thrown, and the default command for TensorFlow's base image would execute at runtime, so it would start a Jupyter notebook IIRC, since we don't have grpc_tensorflow_server.py anymore.

Did I miss something?

If not, we should entirely remove the logic for default PS and force the user to specify a PodTemplateSpec for PS as well.

[ScorpioCPH]

@wbuchwalter Thanks! Addressed your comments, PTAL.