Ensure unattended-updates package is purged

Change-Id: Ib21566a61bf98445dbd555ac17e8e677b2935809
diff --git a/linux/system/single.yml b/linux/system/single.yml
index d7e03e8..ec31a9f 100644
--- a/linux/system/single.yml
+++ b/linux/system/single.yml
@@ -16,6 +16,8 @@
           version: latest
         cloud-init:
           version: purged
+        unattended-updates:
+          version: purged
         mcelog:
           version: latest
       kernel: