devmapper: Use a smaller blocksize for the thin-pool

As per the thin provisioning docs for creating the pool:

   $data_block_size gives the smallest unit of disk space that can be
   allocated at a time expressed in units of 512-byte sectors.
   $data_block_size must be between 128 (64KB) and 2097152 (1GB) and a
   multiple of 128 (64KB). $data_block_size cannot be changed after the
   thin-pool is created. People primarily interested in thin provisioning
   may want to use a value such as 1024 (512KB). People doing lots of
   snapshotting may want a smaller value such as 128 (64KB).

The switch from 512 (which we used before) to 128 (recommended above
for lots of snapshoting) means a simple container creation (based on the
mattdm/fedora:f19 image) adds 1 MB of diskspace rather than 3.6.
This seems more in tune with how docker is typically used.