Silence lxc-unshare warning when removing cgroup

Silence lxc-unshare warning when removing cgroup

If ns cgroup is mounted, then when lxc-unshare runs, the kernel automatically
creates a new cgroup for the task.  So lxc-unshare tries to delete it.  But
if ns cgroup is not mounted, that cgroup does not get created, and now
lxc-unshare spits an error.
Author: Serge Hallyn <serge.hallyn@canonical.com>
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/819319
Forwarded: no

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>