Fix to user module to use self when calling group_exists

library/user

@@ -246,7 +246,7 @@ class User(object):
             cmd.append(self.uid)
 
         if self.group is not None:
-            if not user.group_exists(self.group):
+            if not self.group_exists(self.group):
                 self.module.fail_json(msg="Group %s does not exist" % self.group)
             cmd.append('-g')
             cmd.append(self.group)