By Jose Torres TAPACHULA, Mexico (Reuters) – Thousands of migrants arrived on Sunday in the southern Mexican town Alvaro Obregon to spend the Christmas Eve in a public square without shelter before making their way north to cross the country and reach the border with the United States. On Sunday, the group of mostly Central American and Caribbean migrants had walked 15 kilometers (more than 9 miles) from the southern border city Tapachula to get to Alvaro Obregon, with plans to set off again around 4 a.m. the next day. Migrant rights activist Luis Garcia Villagran urged that migrants must not …