Fix mirror in preseed

Change-Id: Ie36738624d81d4dccd683667f40deabe434c43ac
diff --git a/ubuntu-16.04/http/preseed.cfg b/ubuntu-16.04/http/preseed.cfg
index f30360c..7f8a947 100644
--- a/ubuntu-16.04/http/preseed.cfg
+++ b/ubuntu-16.04/http/preseed.cfg
@@ -1,3 +1,4 @@
+choose-mirror-bin mirror/http/proxy string
 d-i debian-installer/locale string en_US
 d-i debian-installer/language string en
 d-i debian-installer/country EN