Fix risk disaggregation

The was an ugly ordering problem in the code for risk disaggregation (https://bugs.launchpad.net/oq-engine/+bug/1352200). In particular the rupture_ids were extracted from out.output.event_loss_table.keys() in an unspecified order. This as issue because later in the code there is a line `zip(loss_ratios, rupture_ids)` which assumes that the loss_ratios are in the same order as the rupture_ids.
The fix is to extract the rupture_ids from the getters, which list them in the same order used to compute the loss ratios.