Before this patch a container was able to be started
without mounted cgroup. (only a warning was logged)
But the lxc-ps is using the /proc/<pid>/cgroup to identify
the lxc container in which pid is running.
So to be consistent between all lxc command I force an error
if cgroup is not mounted at time of lxc-execute or lxc-start.
Signed-off-by: Michel Normand <normand@fr.ibm.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>