Do not return the list of quadkey by default

[vincentsarago]

cogeo-mosaic/cogeo_mosaic/backends/base.py

Line 77 in 4946b9c

"quadkeys": list(self.mosaic_def.tiles),

This works well on small mosaic, but if you have 10000 quakdey (first mosaicjson file will be quite big) it won't be really efficient.

[kylebarron]

For all backends?

[vincentsarago]

@kylebarron yes, basically we should add an options like we do in dynamoDB https://github.com/developmentseed/cogeo-mosaic/blob/master/cogeo_mosaic/backends/dynamodb.py#L56