Avoid unecessary call to cassandra when using walk.

This commit avoid an unecessary call to cassandra when invoking
the method walk.
According to the documentation cfg.get_range() returns a
tuple of (key, cols).

Change-Id: I4c4c1e62595036e6bea873f7c696f6a1f2a2dfee
Merge "Avoid unecessary call to cassandra when using walk."