By Aaron Maasho ADDIS ABABA (Reuters) - South Sudan#39;s rebels have demanded that Uganda stop supporting government forces as a condition for signing a ceasefire to end fighting that has riven Africa#39;s youngest nation, a spokesman said on Thursday. The president of neighboring Uganda, Yoweri Museveni, admitted for the first time on Wednesday that he was helping South Sudanese President Salva Kiir to fight the rebel forces. Ugandan officials had previously...