WASHINGTON (Reuters) - Presumptive Democratic presidential nominee Hillary Clinton on Thursday welcomed President Barack Obama’s endorsement, telling Reuters in an interview that it “means the world” to her that her former rival has her back in the 2016 campaign. “It is absolutely a joy and an honor that President Obama and I, over the years, have gone from fierce competitors to true friends,” Clinton said.  Clinton and Obama will begin campaigning together next week in Green Bay, Wisconsin. 