[victoriametrics-k8s-stack] Make datasource name configurable or disable provisioning

[D3luxee]

Hi, i just started to look into victoria-metrics-k8s-stack as an replacement for our kube-prometheus-stack. We have a lot of provisioned dashboards that rely on a specific name for the prometheus datasource. At the moment it is not possible to disable the provisioning of the default datasource or change its name.

https://github.com/VictoriaMetrics/helm-charts/blob/master/charts/victoria-metrics-k8s-stack/templates/grafana/datasource.yaml#L1

The current if clause means that i have to disable the deployment of grafana entirely or disable the collection of datasources entirely. It would be nice if i could change the name of the default datasource and disable it without disabling grafana or the sidecar.