gdal2tiles: update prints to use logging package

[zeevdr]

I would like to use the script as part of my code. It would be easier to manage output, if instead of prints you would use the logging package (logger.debug). https://github.com/OSGeo/gdal/blob/35c07b18316b4b6d238f6d60b82c31e25662ad27/swig/python/gdal-utils/osgeo_utils/gdal2tiles.py#L3209

In https://github.com/OSGeo/gdal/blob/35c07b18316b4b6d238f6d60b82c31e25662ad27/swig/python/gdal-utils/osgeo_utils/gdal2tiles.py#L3237 you code use the verbose option to set the logging level.