Fix the site_data table to store one site per row; change gmf_agg to point to site_data

See https://bugs.launchpad.net/oq-engine/+bug/1184603.
Tests are running here: http://ci.openquake.org/job/z_oq-engine_template/16/  (all green)
Notice that a small change in hazardlib is required to run the tests correctly https://github.com/gem/oq-hazardlib/pull/123

For a clean diff, https://github.com/gem/oq-engine/pull/1188 should enter into master