In order for attach to work, the container owner must be able to
write to the tasks file.  Therefore we make the container's cgroup
owned by the container root group, but the container owner uid.
So for the container root to be allowed to create new cgroups, it
needs group write perms.

With this patch, an unprivileged container with an
lxc.mount.auto = cgroup entry entry can run the cgproxy and pass
all cgmanager tests.

Acls would have been another way to do this, but are not yet being
used/exported by cgmanager.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>