Change the dump/restore procedures to work with directories, not tarfiles

We are using the dump/restore procedures to generate the fixtures for the risk tests and it is more convenient to avoid storing tarballs in the repository, otherwise changes in the timestamps will make false differences to appear on git. See https://bugs.launchpad.net/oq-engine/+bug/1221238

I have regenerated the fixtures, see http://ci.openquake.org/job/zdevel_oq-engine/116