Added a test checking the existence of the __init__.py files and fixed the QA test classical/case_15

See https://bugs.launchpad.net/oq-engine/+bug/1334997.
The tests are running here: https://ci2.openquake.org/job/zdevel_oq-engine/451
Notice the needed companion https://github.com/gem/oq-commonlib/pull/22