disable unattended-upgrades for images

Change-Id: Idc4bd64f14046150307746e619d88df079ab95af
diff --git a/ubuntu-16.04/http/preseed.cfg b/ubuntu-16.04/http/preseed.cfg
index 7f8a947..269a2bc 100644
--- a/ubuntu-16.04/http/preseed.cfg
+++ b/ubuntu-16.04/http/preseed.cfg
@@ -37,7 +37,7 @@
 
 d-i pkgsel/include string openssh-server
 d-i pkgsel/install-language-support boolean false
-d-i pkgsel/update-policy select unattended-upgrades
+d-i pkgsel/update-policy select none
 popularity-contest popularity-contest/participate boolean false
 
 d-i pkgsel/upgrade select none