[rollout-operator] make relabelings optional for rollout operator

[lukas-unity]

Rollout operator always creates relabelings when serviceMonitor is enabled, which produces invalid yaml

# Source: rollout-operator/templates/servicemonitor.yaml
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: release-name-rollout-operator
  labels:
    helm.sh/chart: rollout-operator-0.18.1
    app.kubernetes.io/name: rollout-operator
    app.kubernetes.io/instance: release-name
    app.kubernetes.io/version: "v0.19.1"
    app.kubernetes.io/managed-by: Helm
spec:
  selector:
    matchLabels:
      app.kubernetes.io/name: rollout-operator
      app.kubernetes.io/instance: release-name
  endpoints:
    - port: http-metrics
      relabelings:  < --- here
      scheme: http

[Sheikh-Abubaker]

LGTM!