python-lxc: Update for new calls

Add the two new calls to the API and add the new container_path
parameter to the constructor (optional).

This also extends list_containers to support the config_path parameter.
At this point none of the actual tools are changed to make use of those
as we'll probably want to make sure all the tools get the extra option
at once.

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