Force deletion of the `reboot-required` file.

Without this flag, any machines that don't have the file present will cause the task to fail, so we have to remember to pass the `-w` flag to Fabric if we're going to do a reset on a whole class of machines.

Passing `-f` to `rm` instead of using the `warn_only` Fabric option because we don't want to suppress failed attempts  to delete a file when it does exist.