kube-prometheus: extract ServiceMonitors into single files

Starting with Kubernetes 1.6 `kubectl apply` works for TPRs so we can
start using it, therefore using multiple files is no issue.

manifests/prometheus/prometheus-k8s-service-monitor-apiserver.yaml

@@ -0,0 +1,23 @@
+apiVersion: monitoring.coreos.com/v1alpha1
+kind: ServiceMonitor
+metadata:
+  name: kube-apiserver
+  labels:
+    k8s-apps: https
+spec:
+  jobLabel: provider
+  selector:
+    matchLabels:
+      component: apiserver
+      provider: kubernetes
+  namespaceSelector:
+    matchNames:
+    - default
+  endpoints:
+  - port: https
+    interval: 15s
+    scheme: https
+    tlsConfig:
+      caFile: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+      serverName: kubernetes
+    bearerTokenFile: /var/run/secrets/kubernetes.io/serviceaccount/token