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. For example, linode does not support AUFS at the moment.