Explicit ordering to hazard_site query

Added explicit ordering (by id, x, and then y) to HazardSite query used
to construct the SiteCollection.

It seems that, without this explicit ordering, the recent addition of
an index to this caused enough instability to break some tests.

This was causing errors to occur in Celery tasks, which was causing Disaggregation hazard calculations to hang. (See http://ci.openquake.org/job/master_oq-engine/452/console.)

I haven't yet tested to see if this will fix the other failure with the `qa_tests.hazard.event_based.spatial_correlation.case_1.test.EBHazardSpatialCorrelCase1TestCase`, also caught by Jenkins. But my hunch is that this change will fix it.