Ruptures are not read from the database anymore, only written

This is an essential preliminary step to remove the ruptures as pickled objects from the engine database. Before this pull request the event based calculator in the GMF-generation phase was reading the ruptures from the database, now the pickles are not needed. A second pull request will remove them.
See https://bugs.launchpad.net/oq-engine/+bug/1277354 and http://ci.openquake.org/job/zdevel_oq-engine/276