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.

hack/scripts/generate-rules-configmap.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+cat <<-EOF
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: prometheus-k8s-rules
+  labels:
+    role: prometheus-rulefiles
+    prometheus: k8s
+data:
+EOF
+
+for f in assets/prometheus/rules/*.rules
+do
+  echo "  $(basename $f): |+"
+  cat $f | sed "s/^/    /g"
+done