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
I can restore it, I removed it to prevent the temptation of putting some numbers there, given that this a calculation with a single realization. But now nothing bad will happen even if you values for the quantiles, you will get a warning saying that the quantiles could not be computed.