texture_path option still exists in minetest.conf.example but is unusable due to the new mod system. Either remove it from there (bad way) or make it work. For now the only way to change the textures is to download a pack, rename every texture to default_texturename.png and replace the default mod textures. If textures already have to contain mod name in their names, it should be easy to make this option override the default textures of any mod.
It would also be great to add such an option to client menu, but it's non necessary :3