python: Add binding for {get|set}_cgroup_item

python: Add binding for {get|set}_cgroup_item

Updates the binding for the two new functions.

This also fixes some problems with the argument checking of
get_config_item that'd otherwise lead to a segfault.

The python binding for set_cgroup_item and get_cgroup_item are pretty
raw as lxc has little control over the cgroup entries.
That means that we don't try to interpret lists as we do for the config
entries.

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