lxc-ls: Force running against containers without python

When using --nesting, we exec ourselves in the container context, if we
somehow need to dynamically-load modules from there, things break. So
make sure we pre-load everything we may need.

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