frontier-squid requests unnecessary ephemeral-storage when PVCs are used

[rptaylor]

When .Values.SLATE.LocalStorage (side note: that's a bit of a misnomer because PVCs are not necessarily local), a PVC is used for the squid data volume, which makes up most of the storage usage. However that doesn't stop it from requesting ephemeral storage larger than the squid data volume:

https://github.com/slateci/slate-catalog/blob/master/stable/osg-frontier-squid/osg-frontier-squid/templates/deployment.yaml#L71

That means the pod requests a big emptydir , large enough for the squid data cache, but it will not use it since the cache is actually on PVC instead. This wastes disk space and makes the squid pod harder to schedule.
https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/#resource-emphemeralstorage-consumption

[rptaylor]

Values.SLATE.LocalStorage Aside from being a misnomer, the use of a PVC should not be SLATE-specific per se?
Considering also #582 I would suggest a new values section like this for configuring storage-related options:

Storage:
  usePVC: true
  storageClassName: null
  volumeName: foo

or maybe something like this common pattern

persistence:
  enabled: true
  storageClass: null
  volumeName: foo