Better grain handling for telegraf

Replace '-' to '_' in prometheus config dir
Add possibility to add recording rules

Change-Id: Ifc85a95ae62cb0f1b810a3b4f718e8d92f9c6209
diff --git a/README.rst b/README.rst
index 754a94e..627a517 100644
--- a/README.rst
+++ b/README.rst
@@ -17,7 +17,7 @@
     server:
       enabled: true
       dir:
-        config: /srv/volumes/prometheus-config
+        config: /srv/volumes/prometheus
         config_in_container: /opt/prometheus/config
       bind:
         port: 9090
@@ -30,6 +30,10 @@
           cert_name: kubelet-client.crt
           key_name: kubelet-client.key
         etcd: ${etcd:server:members}
+      recording:
+        - name: 'instance:fd_utilization'
+          query: >-
+            process_open_fds / process_max_fds
       alert:
         PrometheusTargetDown:
           if: 'up != 1'
@@ -63,7 +67,7 @@
     alertmanager:
       enabled: true
       dir:
-        config: /srv/volumes/prometheus-config
+        config: /srv/volumes/prometheus
       bind:
         address: 0.0.0.0
         port: 9093