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
Small bikeshed - check err != nil first before create? I know it makes no semantic difference. It seems more idiomatic to check err directly after the api call. Your call on whether to "fix"