ubuntu-cloud-prep: improve overlayfs workaround

the previous 'patch_start' can be vastly simplified now that I better
understand what the bug was.  Instead of wrapping 'start', we only
need to ensure that /etc/init exists inside the overlayfs, so that the
directory that upstart watches is guaranteed to be in the overlay, not
the underlay.

The problem is described under bug 1213925.

Signed-off-by: Scott Moser <smoser@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>