Fix mixin cluster parameter otherwise generated mixins looks like this:
```json
{"type":"prometheus","uid":"${datasource}"},"label":"cluster","name":"job","query":"label_values(etcd_server_has_leader{job=~\".*etcd.*\"}, job)","refresh":2,"type":"query"}]},"time":{"from":"now-15m","to":"now"},"timezone": "`}}{{ .Values.grafana.defaultDashboardsTimezone }}{{`","title":"etcd","uid":"c2f4e12cdf69feb95caa41a5a1b423d9"}`}}
```
where name is job when the variable name used in dashboard queries is cluster.
Signed-off-by: QuentinBisson <quentin@giantswarm.io>
