[generator] Added log info for each stage of MwmStage (#3380)

I think this is useful because MwmStage process is painfully slow.

Signed-off-by: Maciej Sikorski <maciejosikorski@gmail.com>

tools/python/maps_generator/generator/stages_declaration.py

@@ -203,6 +203,7 @@ class StageMwm(Stage):
         ]
 
         for stage in world_stages.get(country, mwm_stages):
+            logger.info('Mwm stage {}: start...'.format(stage.__name__))
             stage(country=country)(env)
 
         env.finish_mwm(country)