ubuntu-image snap unable to locate livecd-rootfs

With the new classic support, we now require livecd-rootfs to be present on the builder host. When running as a snap this does not work currently as we are getting the config file paths by calling dpkg -L which looks for it on the host (and not the snap). We can't depend on livecd-rootfs on the host as ubuntu-image the debian package is now a dependency of livecd-rootfs, meaning you'd have to have both the ubuntu-image snap and deb installed on the system. Besides, we'd anyway want to bundle livecd-rootfs inside the snap anyway.

We need to have a way to inform ubuntu-image to look for the livecd-rootfs config in a selected path.