Fix empty host string when a cluster launched with one single instance

If a cluster is launched on only 1 redis instance, the result of `cluster slots` command may look like

    127.0.0.1:7000> cluster slots
    1) 1) (integer) 0
       2) (integer) 16383
       3) 1) ""
          2) (integer) 7000

The host property is set as an empty string, and this would finally make `socket.getaddrinfo` fail.
It feels wrong to have to explicitly wait this long. Should the server not be up and running when you start it via the Makefile? There should be enough timeouts in there for the server to start.