You can try to use get_host or get_all_hosts function with params = { folder = “your_folder” } (haven’t tried yet) and run kind of a loop foreach found host and run discover_services function with each hostname found.
As far as my (low) dev skills can go, and looking at the webapi code regarding the action_discover_services it can only do to a single host - at least in version 1.5.0p19.
At the moment, the bulk operators are missing inside the API documentation.
Inside the bulk_discovery.py file mentioned from @ricardoftribeiro you find the two possible requests. bulk_discovery_start and bulk_discovery_status described with the parameters