[grafana] Process sidecar configmap/secret label and labelValue with tpl

[a-abella]

This change modifies the sidecar configmap label and labelValue values to be processed with tpl. This is a convenience feature for users with multiple Grafana helm releases installed to the same namespace.

This would allow users to configure a common values file for multiple helm releases to the same namespace similar to:

sidecar:
  dashboards:
    label: 'load-for-{{ .Release.Name }}'

This would allow users to not have to specify a unique .sidecar.xyz.label(Value) for each release's values file, but still retain the ability to dictate which releases should mount which configmaps by referencing the unique release names (or other templated value).

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A note: I attempted to follow the CONTRIBUTING.md guidelines which specified that helm-docs should be generated--I attempted to do so and most of the README was significantly altered, so I instead manually inserted the README changes. If this is improper please let me know and I can follow whatever procedure is expected.