systemd.slice + cgroup enablement fails in multiple ways.

When attempting to configure mesos to use systemd slices on a 'rawhide/f21' machine, it fails creating the isolator:

I0407 12:39:28.035354 14916 containerizer.cpp:180] Using isolation: cgroups/cpu,cgroups/mem
Failed to create a containerizer: Could not create isolator cgroups/cpu: Failed to create isolator: The cpu subsystem is co-mounted at /sys/fs/cgroup/cpu with other subsytems

------ details ------
/sys/fs/cgroup
total 0
drwxr-xr-x. 12 root root 280 Mar 18 08:47 .
drwxr-xr-x. 6 root root 0 Mar 18 08:47 ..
drwxr-xr-x. 2 root root 0 Mar 18 08:47 blkio
lrwxrwxrwx. 1 root root 11 Mar 18 08:47 cpu -> cpu,cpuacct
lrwxrwxrwx. 1 root root 11 Mar 18 08:47 cpuacct -> cpu,cpuacct
drwxr-xr-x. 2 root root 0 Mar 18 08:47 cpu,cpuacct
drwxr-xr-x. 2 root root 0 Mar 18 08:47 cpuset
drwxr-xr-x. 2 root root 0 Mar 18 08:47 devices
drwxr-xr-x. 2 root root 0 Mar 18 08:47 freezer
drwxr-xr-x. 2 root root 0 Mar 18 08:47 hugetlb
drwxr-xr-x. 3 root root 0 Apr 3 11:26 memory
drwxr-xr-x. 2 root root 0 Mar 18 08:47 net_cls
drwxr-xr-x. 2 root root 0 Mar 18 08:47 perf_event
drwxr-xr-x. 4 root root 0 Mar 18 08:47 systemd