By Dawit Endeshaw ADDIS ABABA (Reuters) -Ethiopian Airlines CEO Tewolde Gebremariam, who transformed the airline into Africa’s biggest carrier, said on Wednesday he had resigned to focus on his medical treatment. In an internal memo to employees seen by Reuters he said he had had health issues for almost a year and had been working from the United States for the past five months and was not able to return to the head office any time soon. “I am retiring early due to health issues,” he told Reuters in a text message, echoing the language in the memo which said he would focus on his treatment. I…