Review prometheus server running master nodes

Prometheus should not be allowed in the master IMO since it can kill the node: https://gitlab.cern.ch/kubernetes/automation/releases/cern-magnum/-/blob/17e29a9a774553dcb6b2383f2be4a6c0bd67fc47/values.yaml#L420

masters should run the bare minimum

I am not sure if prometheus made tiller go crazy or not, which blew up the master: INC3151900