Modify system.img build to use ubuntu-core.

Instead of building one from debootstrap. Ubuntu ships a
minimal rootfs called ubuntu-core. This means we don't need
to build it from scratch.

This patch changes the rake task to use a rootfs tarball
either from misc/rootfs.tgz (where it is expected to be cached)
or a user specified file.
Commander::Server does not need to use Thin.