Configure post job the same way as the main kubeflow job

[xyhuang]

Currently the benchmarking kubeflow job is configured through config file, while the post processor is configured through ksonnet with limited functionality (only allows image and commands). Letting the post processor be configured same way as kubeflow job makes it more consistent and also helps to make the post processor fully functional. We can allow post job to be specified in any way that conforms to kubernetes job object.