Fix backup snapshots

The backup tool currently silently fails to write snapshots, due to a missing directory. When one tries to load a backup whose WAL expects a snapshot, starting the cluster fails with `etcd: unknown wal version in data dir`

(This was discussed briefly on [a mailing list thread](https://groups.google.com/forum/?hl=en#!topic/etcd-dev/YEqbp4_zr38))

This change ensures the output directory is made first.