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.
Please restore `startup_nodes` back to a kwarg because in the 0.2.0 refactoring/unstable branch i will implement the host/port args that `StrictRedis` have as an optional way to specify `startup_nodes`. In that solution both `host/port` & `startup_nodes` is optional and if i merge this change i will revert it back in the future anyway. I do like that you moved the checks to an earlier point so that can remain.