[-] BO : fix wrong sitemap url in robots.txt

Hello, default Gsitemap module generates sitemap file with name «{shop_id}_index_sitemap.xml»
While AdminMetaController::generateRobotsFile() updates robots.txt with file «sitemap.xml». 
Fix sm_file definition to correspond «{shop_id}_index_sitemap.xml» form, remove hardcoded «sitemap.xml» from the robots.txt creation procedure.