MEXICO CITY, Sept 27 (Reuters) - Mexico’s president-elect, Andres Manuel Lopez Obrador, assured private energy executives in a closed-door meeting on Thursday their contracts will not be canceled if they meet existing terms, the head of the country’s main oil producers’ association said.Lopez Obrador, who has often expressed skepticism of private sector involvement in Mexico’s oil industry, met for the first time on Thursday with oil and gas executives, striking what was described as a diplomatic tone with them.“The president-elect told us on various occasions . . .