modules overridden by the user are the highest priority, then modules
overridden by the technology. If nothing is overriden, use the defaults
from OPTS (if they exist) or use the requested module_type.
This fixes that custom tech_modules could not be used, if they had a default in
OPTS even if the latter was not overridden by the user.
We don't need extra defaults in the tech_modules, as we now only use them,
if they have been overridden by the tech_module.
Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>