mixins: templatize cluster label name.

[hervenicol]

Is your feature request related to a problem? Please describe.

My Prometheus setup selects data from a specific cluster thanks to a cluster_id label.
The Loki mixins use a hardcoded cluster label.
I would like to be able to configure this when generating the dashboards.

Describe the solution you'd like
mixins are meant to templatize dashboards creation, so I think we could have a 'per_cluster_label' in config.jsonnet file, to select the label name according to our setup.
That would be quite similar to what is done on kubernetes-mixins here: https://github.com/kubernetes-monitoring/kubernetes-mixin#multi-cluster-support

Describe alternatives you've considered
Apart from dirty forking, not much.

Additional context
N/A

[hervenicol]

PR reverted here: #6377
Will create a new (and hopefully better) PR.
@kavirajk , can you please reopen the issue? 🙏