Allow to install prometheus-sli-service in any namespace

[agrimmer]

Currently, the prometheus-sli-service is installed in the namespace keptn.
As a user, I would like to install the prometheus-sli-service in any Kubernetes namespace where Keptn is installed, i.e. with
kubectl apply -f deploy.yaml -n keptn2

This issue can be started after completing issue keptn-contrib/prometheus-service#53