Allow user to use k8s manifest files directly for experiments

[xyhuang]

Right now, we have a kubebench-specific config format that lets user specify the template reference (i.e. ksonnet pkg) and corresponding parameters to use in an experiment. The configurator will automatically generate the job manifests with kubebench-specific mixins. While this is useful, some user might not want to manage their configs in this particular way. We should provide the possibility for users to manage their configs in custom way, which means we can accept raw kubernetes manifest files that specify the kubeflow jobs or post jobs etc. to be run. In this case, the configurator will skip the ksonnet -> manifest generation part, and directly perform the mixins with the raw manifests provided by user.