python3: Support unpriv containers

This removes any existing uid check in the python3 binding and tools,
replacing those by .controllable where appropriate.

Extra checks are also added to make lxc-ls work as a user, returning as
much information as can possibly be retrieved.

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