chart: template and render charts to use service values

The prometheus-operator chart is rendered using the following command:

./charts/render.py prometheus-operator --namespace metalk8s-monitoring charts/prometheus-operator.yaml --service-config grafana
metalk8s-grafana-config --service-config prometheus metalk8s-prometheus-config --service-config alertmanager metalk8s-alertmanager-config
charts/prometheus-operator/ > salt/metalk8s/addons/prometheus-operator/deployed/chart.sls

Service configuration values are passed using `--service-config` argument.
When rendering this chart, we must pass the following services(grafana, prometheus, alertmanager)
including the configmap that holds their respective service & cluster configurations

Closes: #2255