Migrate [prometheus operator] [kube-prometheus] to official stable chart

[goruha]

CoreOs moved prometheus operator charts development to official helm chart

https://github.com/coreos/prometheus-operator/tree/master/helm

https://github.com/helm/charts/tree/master/stable/prometheus-operator

[asiegman]

With coreos now being discontinued, this should probably get prioritized. I'm seeing this using your kube-prometheus helmfile.

...Unable to get an update from the "coreos-stable" chart repository (https://s3-eu-west-1.amazonaws.com/coreos-charts/stable/):
	Failed to fetch https://s3-eu-west-1.amazonaws.com/coreos-charts/stable/index.yaml : 403 Forbidden

Looks like they've already taken stuff down.