Add a ``schema`` command to cmd ctrl to generate schema image. This changeset add the new commande and do some refactoring in cwconfig and schema.py to allow the use of CubicWebNoAppConfiguration with CubicWebSchemaLoader.

#! /bin/sh -e
 
case "$1" in
    remove)
	if [ -x "/etc/init.d/cubicweb-ctl" ]; then
	    invoke-rc.d cubicweb-ctl stop || true
	fi
    ;;
esac
 
#DEBHELPER#
 
exit 0