kube-prometheus: Move service monitors to target folder

With 1d00e0ab ("ServiceMonitor: Enable Prometheus to select ServMon
outside own ns") merged: To emphasize the fact, that ServiceMonitors can
be deployed alongside their targets, this patch moves the Alertmanager,
Kube-State-Metrics, Node-Exporter and Prometheus Operator
Service-Monitor into the respective target folder in the kube-prometheus
project.

jsonnet/node-exporter/node-exporter.libsonnet

@@ -4,4 +4,5 @@
     daemonset:: import "node-exporter-daemonset.libsonnet",
     serviceAccount:: import "node-exporter-service-account.libsonnet",
     service:: import "node-exporter-service.libsonnet",
+    serviceMonitor:: import "node-exporter-service-monitor.libsonnet",
 }