Unify the label keys

[jian-he]

Since we have a common operator, and think the common label keys can also be unified, so that the custom operator will not need to implement these

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This change is 

[terrytangyuan]

/lgtm
This would simplify the implementation.

[johnugeorge]

if same labels are used for all job operators, Will the pod label key-value collide for jobs with same name in the same namespace? (When pods are listed)

[jian-he]

if same labels are used for all job operators, Will the pod label key-value collide for jobs with same name in the same namespace? (When pods are listed)

I think the pod can not be created in the first place, because the pod name needs to be unique, see how the pod name is generated

we can add a label like "job-type=tensorflow" if needed.

Today the tf-operator implementation always list all pods regardless job types and names, which I think can be improved.

[richardsliu]

/lgtm
/approve
+1 for adding the job-type label later.

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