* End of six-month leadership gap at WTO* Okonjo-Iweala plans to prioritise health* WTO expected to decide on next ministerial meetingBy Emma FargeGENEVA, March 1 (Reuters) - The World Trade Organization's first female and first African director-general Ngozi Okonjo-Iweala began work on Monday, ending a six-month leadership void at the global trade watchdog.