Service Deployment v0.18.0 with StatefulSets

[jbeemster]

This update allows us to leverage the existing chart as a StatefulSet with full PVC support baked in. The main difference between these two kinds of deployment is that:

1. Each pod in a StatefulSet has a consistent numerically iterated name (this also applies to upgrade order)
2. When PVCs are enabled rather than creating a single volume for all pods to share we instead shift to a volumeTemplate that lets us create a PVC per pod. These volumes get re-attached to the pod with the same name when upgrades happen / pods move between k8s cluster nodes.
3. The behaviour of PVCs in this mode can also be controlled to either Retain or Delete the volume. By default I have set that on scale_down we Delete the PVC but when we issue a general service removal (helm delete) this retains the volumes (this is the default in the docs) - a user should configure this to suit their own needs!

[istreeter]

Happy to approve this from Data Processing point of view.

[jparavisini]

LGTM!