Jenkins: Add throttle concurrent builds

Change-Id: Ib81abd2b77e5020d2584f59f566e3717df360dab
diff --git a/jessie-amd64-jenkins-master/plugins.txt b/jessie-amd64-jenkins-master/plugins.txt
index 66f69e4..438a841 100644
--- a/jessie-amd64-jenkins-master/plugins.txt
+++ b/jessie-amd64-jenkins-master/plugins.txt
@@ -23,5 +23,6 @@
 ssh-agent:1.19
 ssh-slaves:1.31.0
 timestamper:1.11.3
+throttle-concurrents:2.0.2
 workflow-aggregator:2.6
 ws-cleanup:0.38