Use SSH user name configured for a given target:

When a user is configured for the targeted host name, use it for SSH
authentification instead of the name of current user logged in.

* Modify Remote#user_name to use configured user name.

lib/producer/core/remote.rb

@@ -19,7 +19,7 @@ module Producer
       end
 
       def user_name
-        Etc.getlogin
+        config[:user] || Etc.getlogin
       end
 
       def fs