Unify string formatting

Use one way of string formatting: replace old `%` style with new `str.format`.

Resolve #855.

cdist/message.py

@@ -70,7 +70,7 @@ class Message:
 
         with open(self.global_messages, 'a') as fd:
             for line in content:
-                fd.write("%s:%s" % (self.prefix, line))
+                fd.write("{}:{}".format(self.prefix, line))
 
     def merge_messages(self):
         self._merge_messages()