Fix grafana-dashboards Longhorn dashboard deployment

The following conditional should enable the Longhorn dashboard in the management cluster when longhorn is enabled.

units:
  sylva-dashboards:
    helmrelease_spec:
      values:
        optional_dashboards:
          longhorn:
            enabled: '{{ tuple . "longhorn" | include "unit-enabled" }}'

Normally Longhorn would be deployed in workload clusters so this needs to be changed to check for that