autostart: Define lxc.start.* and lxc.group

First patch in the set of changes required for container autostart.

This commit adds the new configuration keys and parsers that will then
be used by lxc-start and lxc-stop.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ls: Add lxc.start.auto and lxc.group support

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
lxc-ls: Add workaround for nested containers

When accessing nested containers, hardcode the path to their config.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
lxc_start/lxc_stop: Add autostart and group

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