KubeSpawner vision statement: passthrough to pod spec as much as possible

[minrk]

Following #239, I think we should try to stop adding traitlets to KubeSpawner that are simple mappings to entries in the pod spec, and instead work on making passthrough config to Kubernetes a nice experience. As much as possible, I think our goal should be to expose Kubernetes as-is, rather than providing a wrapper around it with different ways to specify all of Kubernetes' existing options, and reserve traitlets for things that need KubeSpawner-level logic (like extra objects, etc.)

That is, things like affinity, etc. should be settable via extra_pod_config and we should provide a nice mechanism for deep-merging merging multiple entries for extra_pod_config, rather than promoting every option to individual traitlets. When met with a proposal to add a new traitlet for a pod attribute, we should try to require that there be a problem setting it with extra_pod_config first, before adding the traitlet.

[tdudgeon]

It's not just the pod spec that needs to be considered here. It would also be useful to pass things through to the PVC definition. For instance I wanted to be able to specify data for pvc.spec.selector.matchLabels to be able to specify which of a set of pre-existing PVs could be used, but it seems that's not currently possible.