XenAPI: Add exit point after JEOS installation

If the user only want to run the installation of Ubuntu - to export the
template as an xva, it just needs to specify a non-empty value for:

EXIT_AFTER_JEOS_INSTALLATION

And the script will exit after the jeos template has been created.

Change-Id: I558e2f2b18ee23c15c7e46e2f7e74543cf26b750