post https://privateurl.com/nirvana/api/v1/units//schedulevast
Returns a VAST tag. Can be an empty tag, or contain an ad to play with play confirmation URL.
The confirmation url, in the VAST response, must be called by all units that were in the unit_uuids.
The confirmation url and error url will contain a "[unit_id]" attribute that must be replaced by the proper unit uuid.