For parity with GCE, we really want to support aufs.
But we previously supported btrfs, so we want to expose that.
Most of the work here is required for aufs, and we let advanced users choose
devicemapper/btrfs if they have a setup that works for those configurations.
We were specifying a region, but naming it as a zone in util.sh
The zone matters just as much as the region, e.g. for EBS volumes.
We also change the config to require a Zone, not a Region.
But we fallback to get the information from the metadata service.