[U.S. President Barack Obama reviews an honor guard with Greek Defense Minister Panos Kammenos upon his arrival in Athens]By Lefteris Papadimas and Renee Maltezou ATHENS (Reuters) - Barack Obama arrived in Greece on Tuesday on his last European tour as U.S. president, and Athens said it held out hope that he will use his clout to help persuade the country's creditors to forgive some of its monumental debt. Obama, who will be replaced in January by Donald Trump, was welcomed by Greek Defence Minister Panos Kammenos at Athens airport at the start of his two-day visit, the first by a U.S. president to Greece since Bill Clinton in 1999.