Grafana dashboards should support the `cluster` variable

[anthr76]

Describe the bug

The grafana dashboard's don't work for prometheus's/grafana's setup for multi-cluster.

Steps to reproduce

    grafana:
      enabled: false
      forceDeployDashboards: true
      sidecar:
        dashboards:
          multicluster:
            global:
              enabled: true
            etcd:
              enabled: true

Expected behavior

A cluster selector for dashboards

Screenshots and recordings

OS / Distro

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Flux version

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Flux check

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Additional context

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