ERP5OOoDocument: use conversion server that has the most chance to work

Until know, in the preference we defined list of uri for conversion server, and then we were trying always in the order defined in the preference. But this is not optimal, when a server start to fail, it's better to try using another one. So sort list of uri in such a way to use first server having the most chance to work.

@xiaowu.zhang , What do you think about this ? Yesterday ERP5-MASTER unit tests were blocked several hours due to cloudooo timeout, so hopefully this change will help.