Stumbled on the fact that LocalDirAllocator.ifExists() is not used
anywhere. The previous usage of this API was in the IsolationRunner
that was removed in MAPREDUCE-2606.
This API doesn't call the confChanged method and hence there is an
uninitialised variable that causes a NullPointerException. So, either
we can fix that, or remove the API if it's not required. This is also
one of the reasons why the IsolationRunner was broken in 1.0.