cgroups: Update systemd to match fs backend

This updates systemd.Apply to match the fs backend by:
* Always join blockio controller (for stats)
* Support CpusetCpus
* Support MemorySwap

Also, it removes the generic UnitProperties in favour of a single
option to set the slice.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)