Using commonlib.readinput.get_source_models

See https://bugs.launchpad.net/oq-engine/+bug/1390348
This is the companion of https://github.com/gem/oq-commonlib/pull/54
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/802
Actually the "generating logic tree realizations" part was moved to post_execute several months ago, I am going to change the docstring.