Use the correct path for the container's cgroup task file.

Also exit out early and cleanly if the container is not running,
and bind-mount /proc/$pid/net with '-n' to keep the entry out
of mtab, else the mtab entry will never go away.
Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>