Make batch scheduler transparent to users

[Jeffwan]

We should extract common logic from kube-batch/volcano, make sure user pass some client by their own based on their choice.

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[zw0610]

Apart from kibe-batch and volcano, I'd also suggest scheduler framework as a choice for users. Is there any progress in this issue? It seems more flexible if we can decouple kube-batch/volcano from kubeflow/common.