fix for the python mutable parameters issue

Simple cleanup:

For reason, see: http://effbot.org/zone/default-values.htm

specifying a parameter with a default parameter of [] can cause unexpected behavior on multiple invocations of the client. I'd actually argue that no default should be given since you can't do anything until you are connected to a cluster. The other way to handle it would be to do:

```
self.startup_nodes = ['127.0.0.1:7000'] if startup_nodes is None else startup_nodes
```

To give it a sane default.