RDC Prometheus plugin

The rdc_prometheus.py is a Prometheus plugin for RDC
The rdc_prometheus_example.yml and prometheus_targets.json are
example Prometheus configuration. If there are multiple compute
nodes, they can be defined at prometheus_targets.json.

Change-Id: I3611b1e8a166f6608351f6e7644808bf72a4d3a0

python_binding/prometheus_targets.json

@@ -0,0 +1,7 @@
+[
+  {
+    "targets": [
+      "localhost:5000"
+    ]
+  }
+]