Solved the problem with UHS

See https://bugs.launchpad.net/oq-engine/+bug/1362649. I also did some cleanup on the hazard map generation mechanism to be consistent with the rest of the code. The tests are green: https://ci.openquake.org/job/zdevel_oq-engine/635
Inside hazard_curves_to_hazard_map there is a loop  `for i, poe in enumerate(poes)`; if poes is left empty in the job.ini file it is converted to an empty list and the maps and uhs are not generated.
Should we raise an error instead, telling the user that it should set the parameter poes if he wants to compute the maps and uhs?