Enables an LXC server's configuration as an inventory source for LXC
containers.
In LXC, containers can be defined with an "lxc.group" configuration
option that is normally used with lxc-autostart -g. Here, we are using
the same option to build Ansible inventory groups.
In addition to being grouped according to their lxc.group entry (or
entries, as LXC allows a single container to be in multiple groups),
we also add all containers (including those with no lxc.group entry)
to the "all" group.
