Not empty folder in NFS errors

Added a more strict condition (https://github.com/oesteban/nipype/blob/bug/fixRemoveNonEmptyDir/nipype/pipeline/engine.py#L1391) when deleting old node folders because with NFS some folders can get locked but empty and the rmtree raises exception. This happens especially when locking files (actually a specific file-locking should be used e.g. http://pythonhosted.org/flufl.lock/).