Fix for #2548

I got an error when running `rethinkdb create`. The problem comes from the new machine name generating code that is recently merged. This bug also breaks some of the tests that rely on the cluster creation.

As there's no point in setting `--port-offset` when making a data directory, we cannot utilize a port number for a machine name. So we simply skip the call to `get_single_int()`, and use the default value of `0` from `port_defaults::port_offset`.