Now the engine logs on a file always

See https://bugs.launchpad.net/oq-engine/+bug/1249759. Now the recommended way to run the engine is without specifying the --log-file option. Then the engine will create a file `calc_<ID>.log` in the `export_dir`. If the user specify a `--log-file` the engine will log there and will not create the file `calc_<ID>.log`. The engine will log on stdout in any case. Just redirect to `/dev/null` if you don't want to see any output.
Tests are running here: https://ci.openquake.org/job/zdevel_oq-engine/904/