fix(disk_layout): Bump up default ROOT volume sizes for btrfs.

btrfs isn't designed for small volumes and can run out of space sooner
than one would expect in our current setup, particularly with docker.
To try to improve the situation always create the filesystem initially
as 2GB instead of 512MB using the default settings: metadata is
duplicated, data is single, not mixed. The mixed setting may have been
partly why our performance can be so poor. For the default vm layout
use 6GB instead of 3GB, about what we use for EC2.
Merge pull request #259 from marineam/disk

fix(disk_layout): Bump up default ROOT volume sizes for btrfs.