[cwconfig] Use imp.load_source instead of exec() to load site_cubicweb.py from apphome Lets us return a module object and go back to the previous code in CubicWebConfiguration._load_site_cubicweb.

[cwconfig] Stop using lgc.modutils to load *config.py and *ctl.py files

[cwconfig] Stop using lgc.modutils to load site_cubicweb.py files config.load_site_cubicweb function now takes a list of cubes (instead of paths) and default to all known cubes.