python3: Add list_containers to C API

This adds a new list_containers function to the python3 binding and a
matching override in __init__.py that adds the as_object parameter.

This should be compatible to the previous pure python implementation
with the advantage of also listing active non-defined containers (fixing
github issue #68).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>