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
#
if [ "$1" = configure ]; then
if which install-docs >/dev/null 2>&1; then
install-docs -i /usr/share/doc-base/cubicweb-doc
fi
fi
#DEBHELPER#
exit 0