base_image_size effectively is the limit of a layer size that can be
created using the devmapper snapshotter. While this will also depend on
the thinpool size itself, something closer to the total image size
(80%?) is more appropriate.
As is, if you try to run an image like elastic, you'll need a much
larger base_image_size than 128MB.
Signed-off-by: Eric Ernst <eric.ernst@intel.com>