Sort track params and user tags when listing races (#659)

This change is affecting the list races subcommand.
It sorts the track parameters in alphabetical order in order to make the output
consistent and also sort the user tags with the same order.

esrally/metrics.py

@@ -1171,7 +1171,8 @@ def race_store(cfg):
 def list_races(cfg):
     def format_dict(d):
         if d:
-            return ", ".join(["%s=%s" % (k, v) for k, v in d.items()])
+            items=sorted(d.items())
+            return ", ".join(["%s=%s" % (k, v) for k, v in items])
         else:
             return None