use centroid instead of shape for node location

You cannot retrieve geo_shape fields from elasticsearch 1.X (see extract from docs below) so this patch uses the centroid instead.

I was not able to run the unit tests but have tested this code within the latest stable release of Osmosis.

```
Due to the complex input structure and index representation of shapes, it is not currently possible to sort shapes or retrieve their fields directly. The geo_shape value is only retrievable through the _source field.
```