mongo: preallocate oplog data files

Preallocate oplog data files. If we don't do this, then mongo is not happy with us when running on big/slow disks. Probably because we pass --nopreallocate to mongod.

Fixes https://bugs.launchpad.net/juju-core/+bug/1327940
`--output` is something added in recent versions of GNU's df. Will need to just plain old "df" output.