Stats only

Added a configuration parameter `individual_curves=false` for the case when the user is interested only on means and quantiles and does not want to save the individual hazard curves (https://bugs.launchpad.net/oq-engine/+bug/1319248). This pull requests also rewrite the configuration/validation mechanism and solves https://bugs.launchpad.net/oq-engine/+bug/1361981.
The tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/622/