add prometheus as last stage

First install Prometheus exporters on new host,
then update monitoring configuration to include new host.

Change-Id: Iba852e7d5a42d79a48481885dd9cde83c5d918f0
diff --git a/openstack-compute-install.groovy b/openstack-compute-install.groovy
index 7d6cf6d..7b9054a 100644
--- a/openstack-compute-install.groovy
+++ b/openstack-compute-install.groovy
@@ -98,6 +98,11 @@
                 }
             }
 
+        stage("Install monitoring") {
+            salt.enforceState(pepperEnv, targetLiveAll, 'prometheus')
+            salt.enforceState(pepperEnv, 'I@prometheus', 'prometheus')
+        }
+
         } catch (Throwable e) {
             // If there was an error or exception thrown, the build failed
             currentBuild.result = "FAILURE"