cgroup: don't include trailing / in cgns_prefix

We could do the math on the consuming side (and indeed, I tried), but it
seems much cleaner to just not include this in the first place so that all
consumers of it don't need to do the same thing.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>