Improvements to DevStack's XenServer scripts

I have ensured:
- template gets re-used on second run
- template includes XenServer tools, and custom user accounts
- take snapshot before first boot, for easy re-run
- make host_ip_iface work with either eth2 or eth3
- make ssh into domU checks looser
- above is all ground work for improved jenkins tests
- added some more comments to make it scripts clearer

Change-Id: I5c45370bf8a1393d669480e196b13f592d29154f
diff --git a/tools/xen/scripts/install-os-vpx.sh b/tools/xen/scripts/install-os-vpx.sh
index fe5e810..7f2f3e6 100755
--- a/tools/xen/scripts/install-os-vpx.sh
+++ b/tools/xen/scripts/install-os-vpx.sh
@@ -19,7 +19,12 @@
 
 set -eux
 
-. /etc/xensource-inventory
+if [ -a /etc/xensource-inventory]
+then
+    . /etc/xensource-inventory
+else
+    . /etc/xcp/inventory
+fi
 
 NAME="XenServer OpenStack VPX"
 DATA_VDI_SIZE="500MiB"