cgmanager: support lxc.mount.auto = cgroup

cgmanager: support lxc.mount.auto = cgroup

If it (or any variation thereof) is in the container configuration,
then mount /sys/fs/cgroup/cgmanager.lower (if it exists) or
/sys/fs/cgroup/cgmanager into the container so it can run a
cgproxy.

Also make sure to clear our groups when we start or attach to a
container.  Else with unprivileged containers we end up with
lots of nogroups listed in /proc/1/status.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>