By Sonali Paul and John Mair MELBOURNE/SYDNEY (Reuters) -Australia will hold a general election on May 21, Prime Minister Scott Morrison said on Sunday, triggering a campaign expected to be fought over cost-of-living pressures, climate change and questions of trust and competence of the major parties. Morrison played up economic uncertainties and security threats in announcing the election, saying this was not the time to hand the reins to an untested opposition Labor leader, Anthony Albanese. “Only by voting for the Liberals and Nationals at this election on May 21 can you ensure a strong eco…