Improve Document Conversion

This MR is plan to make cloudooo conversion more reliable(preparation for the next step: remove cloudooo from ERP5 Software release and use public cloudooo)

What is changed in this MR:

1. System Preference now accept a list of conversion server
2. Add a field in System Preference to define retry count used for network issue case
3. Every call to cloudooo now become a loop of tries : retry count x [url1, url2] only in case of network issue

Test result list

Hardcoded default cloudooo url list for erp5