Add job builds discarder

Change-Id: I7a4a46826b547731b91f2ed1f58927ba4ddd4455
diff --git a/jenkins/client/job/image/debian.yml b/jenkins/client/job/image/debian.yml
index ab17c06..eef4740 100644
--- a/jenkins/client/job/image/debian.yml
+++ b/jenkins/client/job/image/debian.yml
@@ -9,6 +9,11 @@
         build-image-debian-8:
           type: workflow-scm
           concurrent: false
+          discard:
+            build:
+              keep_num: 5
+            artifact:
+              keep_num: 5
           scm:
             type: git
             url: "${_param:jenkins_packer_pipeline}"