Add job builds discarder

Change-Id: I7a4a46826b547731b91f2ed1f58927ba4ddd4455
diff --git a/jenkins/client/job/image/centos.yml b/jenkins/client/job/image/centos.yml
index d5856e4..5358d92 100644
--- a/jenkins/client/job/image/centos.yml
+++ b/jenkins/client/job/image/centos.yml
@@ -9,6 +9,11 @@
         build-image-centos-7:
           type: workflow-scm
           concurrent: false
+          discard:
+            build:
+              keep_num: 5
+            artifact:
+              keep_num: 5
           scm:
             type: git
             url: "${_param:jenkins_packer_pipeline}"