Added boolean attribute flag for controlling use of  docker::aufs.

Docker 0.7 introduces standard linux support by no longer requiring the
use of AUFS (which necessitated a patched Linux kernel). Thus, a boolean
flag (set to true by default) is included as
`node['dokku']['docker']['use_aufs']` and specifies whether or not the
'docker::aufs' recipe will be included during bootstrap. Setting this
flag to false enables docker to be installed on more distributions and
VPS providers.