kube-prometheus: generate manifests without kubectl

For `--dry-run` to work with kubectl a Kubernetes cluster's apiserver is
actually used, which is unnecessary for generating these manifests. This
approach also allows further customization, such as adding labels to the
generated manifests.

manifests/prometheus/prometheus-k8s.yaml

@@ -10,6 +10,10 @@ spec:
   serviceMonitorSelector:
     matchExpression:
     - {key: k8s-apps, operator: Exists}
+  ruleSelector:
+    matchLabels:
+      role: prometheus-rulefiles
+      prometheus: k8s
   resources:
     requests:
       # 2Gi is default, but won't schedule if you don't have a node with >2Gi