xenapi - cleanup

Add error on unitialised variable to the scripts. This way some issues
were identified. Also modify README with fresh variables. The patch
includes:
 - Add SWIFT_HASH to the README
 - Add XENAPI_CONNECTION_URL to the README
 - Add VNCSERVER_PROXYCLIENT_ADDRESS to the README
 - Introduce UBUNTU_INST_IFACE which is the OpenStack VM interface used
   for the netinstall. It defaults to eth3. Previously this parameter
   was set by the combination of HOST_IP_IFACE and the undocumented
   NETINSTALL_IP
 - get rid of NETINSTALL_IP
 - xenrc includes CLEAN_TEMPLATES
 - xenrc no longer tries to change directory
 - remove chrooting from prepare_guest.sh (STAGING_DIR was always / )
 - remove DO_TGZ variable from prepare_guest.sh
 - use arguments to call prepare_guest.sh, instead of env vars
 - Fix backslash escaping in prepare_guest_template.sh

NOTE: networking is about to be addressed in a separate change.

Related to blueprint xenapi-devstack-cleanup

Change-Id: Ie9a75321c7f41cc9a0cc051398d1e6ec2c88adfa
diff --git a/tools/xen/install_os_domU.sh b/tools/xen/install_os_domU.sh
index 7c3b839..bcea939 100755
--- a/tools/xen/install_os_domU.sh
+++ b/tools/xen/install_os_domU.sh
@@ -1,15 +1,13 @@
 #!/bin/bash
 
-# This script is a level script
-# It must be run on a XenServer or XCP machine
+# This script must be run on a XenServer or XCP machine
 #
 # It creates a DomU VM that runs OpenStack services
 #
 # For more details see: README.md
 
-# Exit on errors
 set -o errexit
-# Echo commands
+set -o nounset
 set -o xtrace
 
 # Abort if localrc is not set
@@ -31,13 +29,12 @@
 # xapi functions
 . $THIS_DIR/functions
 
-
 #
 # Get Settings
 #
 
 # Source params - override xenrc params in your localrc to suit your taste
-source xenrc
+source $THIS_DIR/xenrc
 
 xe_min()
 {