python-rados: refactor class Rados a little bit

just to improve the readability.

and i believe we don't need to check the existence of `self.state` before referencing it in `shutdown()`. it should be always there, as in `__init__()`

```python
        ret = run_in_thread(self.librados.rados_create2,
                            (byref(self.cluster), c_char_p(clustername),
                            c_char_p(name), c_uint64(flags)))

        if ret != 0:
            raise Error("rados_initialize failed with error code: %d" % ret)
        self.state = "configuring"
```

and one (should) never deletes this attribute.