cgroup file relocation

on my box the location of the cgroup directory has changed from $CID/ to docker-$CID.scope/, like this:
$ find /sys/fs/cgroup/devices -name "docker-*"
/sys/fs/cgroup/devices/system.slice/docker-bfac064c180363bdfe406a563f6b2367fedcb580cae623a7de73bc73b7e78c88.scope

this patch lets me use pipework with these location changes.

Docker version 0.10.0-dev, build 566d49c
Linux 3.13.7-1-aufs_friendly #1 SMP PREEMPT Wed Mar 26 22:01:25 MDT 2014 x86_64 GNU/Linux