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

tools/xen/install_os_domU.sh

@@ -244,6 +244,11 @@ else
     vm_uuid=$(xe vm-install template="$TNAME" new-name-label="$GUEST_NAME")
 fi
 
+if [ -n "${EXIT_AFTER_JEOS_INSTALLATION:-}" ]; then
+    echo "User requested to quit after JEOS instalation"
+    exit 0
+fi
+
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