Machine name cannot be generated on "rethinkdb create"

I got an error when running `rethinkdb create`. The problem comes from the new machine name generating code #2548 that is recently merged, which was intended to fix #236. 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`.