We were splitting the aufs storage into docker & kubernetes areas, but
the kubernetes area was filling up very quickly because empty volumes
went on there, and I had originally not sized it big enough for that.
Instead, create one volume for both so they can share space freely. We
can't do this for devicemapper, but that configuration seems to be
deprecated by Docker anyway.
If we're going to have a persistent disk on /mnt/master-pd, it seems risky
sometimes to have /mnt be a mounted volume.
A new consistent approach: we mount volumes under /mnt/<name>.
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.
