Set build-days-to-keep to 30 by default.

Build logs were being removed after 7 days which is too short.

Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>
Change-Id: I29e6be114a05878126f1b1fa38bc747f92386746
diff --git a/jjb/defaults.yaml b/jjb/defaults.yaml
index 24795f0..34b92a8 100644
--- a/jjb/defaults.yaml
+++ b/jjb/defaults.yaml
@@ -9,6 +9,10 @@
     jenkins-ssh-credential: jenkins
     lftools-version: <1.0.0
 
+    # build discards
+    build-days-to-keep: 30
+
+    # pattern for build files to archive
     archive-artifacts: ''
 
     # most jobs invoke docker