Check for existence before deletion, not for type:file

This check was always failing as the type was a directory and not a file. Using `-e` means that we check for any type.
Stop the service before deleting the files we depend on

Now we are actually deleting the files, it is better to stop the service before deleting the files from under a running process. I've renamed the `Restart` task to `Service` as it is no longer being used simply for restarting.
Merge pull request #190 from guardian/sh-native-apps-task-update

Check for existence before deletion, not for type:file
Cull instances in order based on availability zone