Move the cluster name from ConfigWriter to SimpleStore.
Reason: if left ConfigWriter, nodes don't know to which cluster they belong. This will bite us later when we'll revisit node join operation. Cons: we lose the cluster name from the config file, which means a standalone "cluster.data" is a little bit harder to analyze. But I have a plan for this which will follow shortly. Implementing this will also allow us to cleanly fix "gnt-cluster getmaster". Reviewed-by: imsnah