Data-nodes should be formatted when the name-node is formatted.

The upgrade feature (削除) HADOOP-702 (削除ここまで) requires data-nodes to store persistently the namespaceID
in their version files and verify during startup that it matches the one stored on the name-node.
When the name-node reformats it generates a new namespaceID.
Now if the cluster starts with the reformatted name-node, and not reformatted data-nodes
the data-nodes will fail with
java.io.IOException: Incompatible namespaceIDs ...

Data-nodes should be reformatted whenever the name-node is. I see 2 approaches here:
1) In order to reformat the cluster we call "start-dfs -format" or make a special script "format-dfs".
This would format the cluster components all together. The question is whether it should start
the cluster after formatting?
2) Format the name-node only. When data-nodes connect to the name-node it will tell them to
format their storage directories if it sees that the namespace is empty and its cTime=0.
The drawback of this approach is that we can loose blocks of a data-node from another cluster
if it connects by mistake to the empty name-node.