Use old yml file for down to clean old containers

[jerryleooo]

Currently such situations exists:

we scale up one service to 5 instances and later we want to scale down it to 1 instances. But with https://github.com/lock-free/flexdeploy/blob/master/src/deploy.js#L84 we will use the same yml file for down && up, which will leave the other 4 instances alive. A better way maybe copy the old yml file to another file, use it to down services and use the new yml file to up services.