lxc-ls is currently exiting with return code 1 when called with --help
or when called on a system without containers.

This behaviour isn't documented in the manpage and isn't terribly intuitive.
It's been the source of quite a few weird failures in scripts running with
set -e.

As a user calling --help is a voluntary action, lxc-ls should exit 0.

Also, as lxc-ls's goal is solely to list containers, showing an error and
exiting with return code 1 when there's no container seems counter-intuitive
and error-prone.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>