xenapi: set dhcp timeout on VM installation

Set the DHCP timeout to 120 seconds during virtual machine installation.
Some users failed to run devstack, due to a low DHCP timeout setting.
The default value is 60 seconds. This change sets the value to 120 secs,
that should give enough time for most people.

Change-Id: I15fde45ed0d005c1a8621134eee6c3c338b5be5d
diff --git a/tools/xen/devstackubuntupreseed.cfg b/tools/xen/devstackubuntupreseed.cfg
index c559b1e..6a1ae89 100644
--- a/tools/xen/devstackubuntupreseed.cfg
+++ b/tools/xen/devstackubuntupreseed.cfg
@@ -34,7 +34,7 @@
 
 # If you have a slow dhcp server and the installer times out waiting for
 # it, this might be useful.
-#d-i netcfg/dhcp_timeout string 60
+d-i netcfg/dhcp_timeout string 120
 
 # If you prefer to configure the network manually, uncomment this line and
 # the static network configuration below.