Update Lua API

Add [gs]et_config_path from API to Lua binding. Add additional optional
parameter to container_new(). Add tests for these new Lua API bindings.
Commit 2a59a681 changed the meaning of lxc_path_get() in the binding,
causing lua script breakage. Reinstate original behavior of
lxc_path_get() and rename it to lxc_default_config_path_get() to make
its intent clearer.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>