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 eth2. Previously this parameter
   was set by the combination of HOST_IP_IFACE (which defaults to eth3)
   and the undocumented NETINSTALL_IP
 - xenrc inscludes CLEAN_TEMPLATES
 - xenrc no longer tries to change directory
 - prepare_guest.sh - cleanup non-used chroots, parameters
 - prepare_guest.sh is using arguments instead of env vars

Related to blueprint xenapi-devstack-cleanup

Change-Id: Ie9a75321c7f41cc9a0cc051398d1e6ec2c88adfa