WIP: Refactoring of Conversion Server for wkhtmltopdf support
@nexedi @yusei has asked me to create a report for new VAT ratio and we wanted to evaluate wkhtmltopdf. I noticed that there was no existing Transform to allow direct HTML to PDF conversion for Cloudooo (currently with wkhtmltopdf) so I have refactored the code for connecting to Cloudooo to move it away from ERP5OOo (see the commit messages for further explanations of the changes). For now, I have only kept backward compatibility and not renamed the code in the whole repository (that's why the commit is still WIP) as I wanted to get some feedbacks before.