Add a mechanism to populate the db from CSV files, without the need to run a fake calculation

This is a first step for solving the issue with the current models_test.py, see https://bugs.launchpad.net/oq-engine/+bug/1167310; the tests will be addressed
in a future pull request.

The current work is also a first step needed for https://bugs.launchpad.net/oq-engine/+bug/1167693, even if that is a more complex case and will require a smarter approach.