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
Why did you remove it? In general if we leave a key empty, this means that nothing is going to be computed.