Allow to configure report convert command
When deploying trytond
in a scalable environment (like k8s), we scale it by launching new instances. But as each instance must be able to launch LibreOffice for the conversion, the scaling can be done only with instances with at least 2G of memory.
This can be a lot of resource reserved for rare usage.
So it will be good to be able to delegate the conversion to a dedicated host. For that we need to be able to configure the command line used for the conversion.