Fix mirror in preseed

Change-Id: Ie36738624d81d4dccd683667f40deabe434c43ac
diff --git a/ubuntu-14.04/http/preseed-lvm.cfg b/ubuntu-14.04/http/preseed-lvm.cfg
index 859c20c..81c7bac 100644
--- a/ubuntu-14.04/http/preseed-lvm.cfg
+++ b/ubuntu-14.04/http/preseed-lvm.cfg
@@ -1,3 +1,4 @@
+choose-mirror-bin mirror/http/proxy string
 d-i debian-installer/framebuffer boolean false
 d-i debconf/frontend select noninteractive
 d-i base-installer/kernel/override-image string linux-server