Simplify schema

Addresses https://bugs.launchpad.net/oq-engine/+bug/1246379

The motivation of this change comes from the need of moving generic Input/ModelContent tables to the oq-platform and assume that oq-engine-server is always on the same machine where the controller node (of oq-engine) is installed.

CI build is green: http://ci.openquake.org/job/zdevel_oq-engine/165/