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