Consider removing the default `super.withImagePullPolicy('IfNotPresent')` for containers

Since [k8s now sets the imagePullPolicy automatically upon creation](https://kubernetes.io/docs/concepts/containers/images/#imagepullpolicy-defaulting), setting it to `IfNotPresent` for the container to be valid is not required anymore.
https://github.com/grafana/jsonnet-libs/blob/98c3060877aa178f6bdfc6ac618fbe0043fc3de7/ksonnet-util/grafana.libsonnet#L19

Overriding things like this can be cumbersome with jsonnet so it would be nice to not be forcing this on all containers created using the library. Either removing this default entirely or just making the default pull policy value configurable "globally" would be greatly appreciated.