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.
debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl