Make TF serving component more readable and extendable

[lluunn]

https://github.com/kubeflow/kubeflow/blob/master/kubeflow/tf-serving/tf-serving.libsonnet
It's currently not very readable, and adding other features will make it worse (e.g. request logging).

Some previous discussion in #376 .

Here are some example packages: https://github.com/ksonnet/parts/tree/master/incubator
From this I think the general pattern is for each component, define

1. default param
2. base
3. different extensions

We can utilize ksonnet mixins.

[jlewi]

I think we should avoid mixins.

The direction we are trending in see (#564) is having ks generate emit a complete object that the user can then modify to customize as needed, with some common fields (e.g. parameter) exposed as parameters.

Furthermore, we should see if we can further simplify things by leveraging YAML support in ksonnet rather then representing prototypes using jsonnet.

[jlewi]

Given #1229 I think the use case is to support things like injecting a config map and sidecar for request logging.

I think there are two patterns we should explore

1. Using custom admission controllers
2. Using overlays
   • I think this was introduced by kustomize
   • But I believe its supported in ksonnet with the "+" operator

[lluunn]

#1589 is the new pattern. We can improve on that