Merge pull request #3890 from edx/feanil/add_throttle_conurrents_plugin

Add the throttle concurrents plugin to jenkins master.

playbooks/roles/jenkins_master/defaults/main.yml

@@ -73,6 +73,7 @@ jenkins_plugins:
     - { name: "ssh-credentials", version: "1.11" }
     - { name: "ssh-slaves", version: "1.9" }
     - { name: "shiningpanda", version: "0.23" }
+    - { name: "throttle-concurrents", version: "1.9.0" }
     - { name: "tmpcleaner", version: "1.1" }
     - { name: "token-macro", version: "1.10" }
     - { name: "timestamper", version: "1.5.15" }