* Bella ethnic group has served Tuaregs for centuries
* Campaigners call arrangement slavery
* Tuareg leader in camp says his group unfairly singled out
By Misha Hussain
GOUDEBOU, Burkina Faso (Thomson Reuters Foundation) - Ten-year-old Atahib goes to school in the Goudebou refugee camp in the savannah of Burkina Faso, not hoping to become a teacher or a doctor like his classmates. He ...