lxc: tests: use busybox template rather than ubuntu

to speed up tests where it doesn't matter

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
support lxc.mount.auto for get_config_item and clear_config_item

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