Add job builds discarder

Change-Id: I7a4a46826b547731b91f2ed1f58927ba4ddd4455
diff --git a/jenkins/client/job/image/ubuntu.yml b/jenkins/client/job/image/ubuntu.yml
index 8d7e1de..e4a8251 100644
--- a/jenkins/client/job/image/ubuntu.yml
+++ b/jenkins/client/job/image/ubuntu.yml
@@ -88,6 +88,13 @@
         build-image-ubuntu-16-04:
           type: workflow-scm
           concurrent: false
+          discard:
+            build:
+              keep_num: 5
+              keep_days: 5
+            artifact:
+              keep_num: 6
+              keep_days: 6
           scm:
             type: git
             url: "${_param:jenkins_packer_pipeline}"