Detect existing var lib docker on hd

#23 - it works, and it changes the on HD format to use ``/var/lib/docker`` and ``/var/lib/boot2docker`` for both empty disks and ones with OS's on them.

for now, I've added ``/var/lib/boot2docker/etc/hostname`` and ``/var/lib/boot2docker/etc/docker-daemon`` on the HD to allow us to customised the hostname (I have 2 or 3 of these atm) and the daemon commandline

it looks for ``/vm`` and if it finds it uses the old layout, plus the customisable files.