YARN_PID_DIR should be visible in yarn-env.sh

Currently I see YARN_PID_DIR only show in yarn-deamon.sh which is supposed not the place for user to set up enviroment variable. IMO, yarn-env.sh is the place for users to set up enviroment variable just like hadoop-env.sh, so it's better to put YARN_PID_DIR into yarn-env.sh. ( can put it into comment just like YARN_RESOURCEMANAGER_HEAPSIZE )